So earlier today I was playing Gmod, and i decided to join a HL2RP server and while it was loading some things I went to the bathroom, when I came back Gmod was just a complete black screen, and I alt+tab'd out of it and hovered my mouse of the icon then the window-screen, and it showed this box that.. I can't remember but for some reason I remember it having some-type of windows-classic theme, but anyways, it said something like "Illegal asset terminated" and the buttons (which i can barely remember) which one said "Debug Info" and they had a box by them that had the number "1" by them and it seemed like you could click them and change the numbers, but I didn't, so I Ctrl+Alt+Del'd and exited Gmod using it and then went onto Garry's Mod again and went onto Singleplayer and it worked just fine. I haven't tried joining a server yet, though.
Also, it wouldn't be a rogue anti-virus because I haven't downloaded any single thing in the past few days (Except for a steam update which was for CS;S though), and i'm sure it would've blocked any internet-thing if it was.