Gmod doesn't work!!! somebody help!!!

I bought Garry’s mod and installed it. When I press launch game everything works fine; he starts loading
screen turns into Gmod image but then the screen turns black and it stops running and I get the error:

Hl2.exe doesn’t work anymore

What to do!? :S :’(

Specs?

Specs please.

[editline]09:43AM[/editline]

Argh, ninja’d.