I am running Kubuntu 15.04 with the Plasma 5 desktop. Yesterday I tried to install Stata 12 on my machine as I did many times before with previous Ubuntu versions. After finishing the installation routine I tried to start Stata, but my system complained that libgtksourceview1.0-0 was missing. Because libgtksourcview1.0-0 is no longer in the archives, I installed libgtksourceview2.0-0 and created a symbolic link like stated here.

I logged out and in again, started Stata: libgnomeprint2.0-0 was missing. I followed the advice in the link above and added

deb http://old-releases.ubuntu.com/ubuntu saucy main restricted universe multiverse

to my sources.list and installed libgnomeprint2.0-0.

After logging out and in again, my Plasma desktop was broken. A black screen with a blank with bar on the bottom. The Alt+F2 command prompt appears as well, but has no effect.

I tried to remove and purge libgnomeprint and even libgtksourceview, but it had no effect. So how can I get Plasma working again and is there a way to get Stata running without causing conflicts with Plasma?

The problem is not libgnomeprint, but that you mixed in packages from an old release and now end up with a will mix of library versions.
remove the saucy source and call "apt-get update; "apt-get upgrade" to get back to a clean system installation.

symlinking different major versions of libraries is *extremely* fragile, don't do that. Even if you get the application to run, there's great potential for random crashes.
Fetch the actually required library (and it's deps) move it into the stata instalation dir (likely in /opt) and export LD_LIBRARY_PATH accordingly.