Crash??

So… I double click on the shortcut for Garrys Mod and it begins to open. The main screen comes up, still loading. then it says hl2.exe has stopped working and there is a black square ( about 1/4 of the screen for a few seconds) which I assume is the game closing. I also re downloaded… what now?

(on my friends pc, but he doesnt have an account here.)