I changed the resolution while playing the game and it crashed. Now i cant start the game anymore. Any ideas?

Maybe editing the gothic.ini (should be somewhere in the Gothic folder) could help.
Look for

zVidResFullscreenX=
zVidResFullscreenY=

fill in something your machine should definitely be able to run (like 800x600), save and close.

Have a look and if the above shouldn't work, repost your question in the Gothic-specific subforum here:
https://www.gog.com/forum/gothic_series#1572737945