Computer screen suddenly wont work with Gmod after about 3 months?

I haven’t played Gmod in about 3-4 months. About 5 months ago I got a new computer screen, and Gmod worked fine with it (I got to increase the ratio and up the graphics with a new card too) but now whenever I launch Gmod the screen goes black for a minute, then a message appears saying “Input not supported” and I have to turn off the computer, since Ctrl-Alt-Del just crashes my comp. Any ideas why?

Specs please.

System Specs and make sure your drivers are up to date.

Same thing happening for me with an acer X23W monitor and a GeForce GT 430 Graphics card running windows XP 64-bit and i updated my graphics card a few days ago