Recently, I ran into this problem that keeps me at the command line prompt. I have been using KDE on my Debian system for a couple months now, but in the last few days, I have been getting "display not found", "no $DISPLAY", or "can't load display "" " messages. I have a clue that it is related to an aptitude call I ran to set up qemu and kvm virtualization managers. I carelessly selected "yes" on one of the prompts, and then saw messages saying that all my "k" programs were getting removed. Obviously the question had something to do with removing KDE, though I have no clue why it would want to do that. So I reboot and find a command prompt login and display not found messages. What I did after that was "sudo apt-get install kde-full", and KDE reinstalled. I then rebooted the system.

This worked one time. My desktop and settings were as if nothing happened. I was relieved. I had to run nvidia-settings to get the display back to 1368x768, which I normally use. I know this program modifies X settings. I also tried to install kvm again. No messages about removing programs showed up, and I thought I was in the clear. Things seemed to be working normally. After powering down for the day, I come back the next day and it was back to the command prompt. Removing and reinstalling KDE no longer works. I get the same error message about a null display string.

I'll dump a few of the suspected files for more information.

First, here's /var/log/Xorg.0.log

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[   667.858]
X.Org X Server 1.16.4
Release Date: 2014-12-20
[   667.858] X Protocol Version 11, Revision 0
[   667.858] Build Operating System: Linux 3.16.0-4-amd64 x86_64 Debian
[   667.858] Current Operating System: Linux debian 3.2.0-4-amd64 #1 SMP Debian 3.2.65-1+deb7u2 x86_64
[   667.858] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-3.2.0-4-amd64 root=UUID=885001a1-46a4-4cc2-9ab6-24ea17a27a70 ro quiet
[   667.859] Build Date: 11 February 2015  12:32:02AM
[   667.859] xorg-server 2:1.16.4-1 (http://www.debian.org/support)
[   667.859] Current version of pixman: 0.32.6
[   667.859]    Before reporting problems, check http://wiki.x.org
   to make sure that you have the latest version.
[   667.859] Markers: (--) probed, (**) from config file, (==) default setting,
   (++) from command line, (!!) notice, (II) informational,
   (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
[   667.860] (==) Log file: "/var/log/Xorg.0.log", Time: Sun Jun 14 17:06:26 2015
[   667.861] (==) Using config file: "/etc/X11/xorg.conf"
[   667.861] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[   667.861] (==) ServerLayout "Layout0"
[   667.861] (**) |-->Screen "Screen0" (0)
[   667.861] (**) |   |-->Monitor "Monitor0"
[   667.862] (**) |   |-->Device "Device0"
[   667.862] (**) |-->Input Device "Keyboard0"
[   667.862] (**) |-->Input Device "Mouse0"
[   667.862] (**) Option "Xinerama" "0"
[   667.862] (==) Automatically adding devices
[   667.862] (==) Automatically enabling devices
[   667.862] (==) Automatically adding GPU devices
[   667.862] (WW) The directory "/usr/share/fonts/X11/cyrillic" does not exist.
[   667.862]    Entry deleted from font path.
[   667.862] (==) FontPath set to:
   /usr/share/fonts/X11/misc,
   /usr/share/fonts/X11/100dpi/:unscaled,
   /usr/share/fonts/X11/75dpi/:unscaled,
   /usr/share/fonts/X11/Type1,
   /usr/share/fonts/X11/100dpi,
   /usr/share/fonts/X11/75dpi,
   built-ins
[   667.862] (==) ModulePath set to "/usr/lib/xorg/modules"
[   667.862] (WW) Hotplugging is on, devices using drivers 'kbd', 'mouse' or 'vmmouse' will be disabled.
[   667.862] (WW) Disabling Keyboard0
[   667.862] (WW) Disabling Mouse0
[   667.862] (II) Loader magic: 0x7fcc8e37dd80
[   667.862] (II) Module ABI versions:
[   667.862]    X.Org ANSI C Emulation: 0.4
[   667.862]    X.Org Video Driver: 18.0
[   667.862]    X.Org XInput driver : 21.0
[   667.862]    X.Org Server Extension : 8.0
[   667.864] (--) PCI:*(0:1:0:0) 10de:0fc6:196e:0973 rev 161, Mem @ 0xfd000000/16777216, 0xc0000000/268435456, 0xd0000000/33554432, I/O @ 0x0000e000/128, BIOS @ 0x????????/524288
[   667.864] (II) LoadModule: "glx"
[   667.864] (II) Loading /usr/lib/xorg/modules/linux/libglx.so
[   667.881] (II) Module glx: vendor="NVIDIA Corporation"
[   667.881]    compiled for 4.0.2, module version = 1.0.0
[   667.881]    Module class: X.Org Server Extension
[   667.881] (II) NVIDIA GLX Module  340.65  Tue Dec  2 09:10:06 PST 2014
[   667.881] (II) LoadModule: "nvidia"
[   667.881] (II) Loading /usr/lib/xorg/modules/drivers/nvidia_drv.so
[   667.882] (II) Module nvidia: vendor="NVIDIA Corporation"
[   667.882]    compiled for 4.0.2, module version = 1.0.0
[   667.882]    Module class: X.Org Video Driver
[   667.882] (II) NVIDIA dlloader X Driver  340.65  Tue Dec  2 08:47:36 PST 2014
[   667.882] (II) NVIDIA Unified Driver for all Supported NVIDIA GPUs
[   667.882] (--) using VT number 7

[   667.895] (II) Loading sub module "fb"
[   667.895] (II) LoadModule: "fb"
[   667.895] (II) Loading /usr/lib/xorg/modules/libfb.so
[   667.895] (II) Module fb: vendor="X.Org Foundation"
[   667.895]    compiled for 1.16.4, module version = 1.0.0
[   667.895]    ABI class: X.Org ANSI C Emulation, version 0.4
[   667.896] (WW) Unresolved symbol: fbGetGCPrivateKey
[   667.896] (II) Loading sub module "wfb"
[   667.896] (II) LoadModule: "wfb"
[   667.896] (II) Loading /usr/lib/xorg/modules/libwfb.so
[   667.896] (II) Module wfb: vendor="X.Org Foundation"
[   667.896]    compiled for 1.16.4, module version = 1.0.0
[   667.896]    ABI class: X.Org ANSI C Emulation, version 0.4
[   667.896] (II) Loading sub module "ramdac"
[   667.896] (II) LoadModule: "ramdac"
[   667.896] (II) Module "ramdac" already built-in
[   667.897] (EE) NVIDIA: Failed to initialize the NVIDIA kernel module. Please see the
[   667.897] (EE) NVIDIA:     system's kernel log for additional error messages and
[   667.897] (EE) NVIDIA:     consult the NVIDIA README for details.
[   667.897] (EE) No devices detected.
[   667.897] (EE)
Fatal server error:
[   667.897] (EE) no screens found(EE)
[   667.897] (EE)
Please consult the The X.Org Foundation support
    at http://wiki.x.org
 for help.
[   667.897] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[   667.897] (EE)

/etc/X11/xorg.conf, which seems to look fine.

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# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 304.48  (pbuilder@cake)  Wed Sep 12 10:54:51 UTC 2012

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 304.88  (pbuilder@cake)  Wed Apr  3 08:58:25 UTC 2013

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "0"
EndSection

Section "Files"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"

    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "ORN ORION"
    DisplaySize     409    230
    HorizSync       15.0 - 68.0
    VertRefresh     54.0 - 76.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 650"
EndSection

Section "Screen"

# Removed Option "metamodes" "nvidia-auto-select +0+0"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "DPI" "85x85"
    Option         "Stereo" "0"
    Option         "metamodes" "1360x768 +0+0"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection


/etc/X11/xinit/xinitrc, which seems a little empty.

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#!/bin/sh

# /etc/X11/xinit/xinitrc
#
# global xinitrc file, used by all X sessions started by xinit (startx)

# invoke global X session script
. /etc/X11/Xsession

#exec startkde                       (I commented this out trying to fix said problem)

xhost +
xrandr --output HDMI-0 --fbmm 409x230


/etc/X11/xinit/xserverrc

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#!/bin/sh

xrandr --output HDMI-0 --fbmm 409x230
xhost +

exec /usr/bin/X -nolisten tcp "$@"

409x230 seems like a little weird (small), but I am not familiar with the x*rc files.