I installed KDE yestreday (not me presonally unfortunately but someone I know) and was able to go into the system with no login screen.
When I turned it on today there is a login screen. It let's me type my password but then happens. I press enter and it does nothing, not getting stuck, just nothing. It still let's me write letters in the password box, so it's not frozen.
It used to show me a virtual keyboard but I deleted libqtvirtualkeyboardplugin.so as written here:
https://bugs.kde.org/show_bug.cgi?id=415130
I'm adding pictures of the login screen:
https://ibb.co/JBpVZB8
https://ibb.co/Kyqmr3K

I'm pretty new to linux so assume I don't know anything, I'll try to learn anything you say though.

Do you know what operating system is installed? KDE/Plasma is just the graphical interface set up on a running linux distribution.

Try to type letters of your password like they are on us layout keyboard → Wikipedia - US Layout
Password is case sensitive, PassWord is another password than passWord. Or just ask this guy what he set up there

To my understanding my operating system is debian.
Also my password is numeric so no problems with capitalization. I do know about capitalization in linux btw (and in passwords in general lol).
As i said, the problem is not with the password, it's that I can't even press anywhere to try to sign in, and return don't seem to work. I can still get to tty2,3 etc. so maybe something can be done from there.

Okay. You can try to login on a virtual terminal by pressing Ctrl-Alt-F2 (or 1-7 by default, one is your current instance of sddm, the login manager you see) and try to login there. The password typing is not visual shown, so you have to type it "blind".

If this works, you can locate errors looking at dmesg output or journalctl -b -xep 0..4 on a systemd-based distro (most have nowadays). Perhaps you get an error message which will help tracking down the problem. If login does not work, either your username or password does not match.

I managed to fix it by doing the following:
1. apt-get full-upgrade, then it told me to autoremove some stuff.
2. apt-get autoremove whatever it told me (sorry, I wasn't paying attention as I should have)
3. reboot
4. apt install sddm
5. startx

Then for some reason I had to reinstall plasma-desktop (apt install plasma-desktop).
That solved the problem for me, and I set it to sign-in automatically next time from the system settings.

This can be marked as solved.
Thanks anyway.