gmod crashing on opening

I have been trying to play Garrys mod for the past couple of days and it will just crash. It comes up and says hl2.exe has stopped working windows will notify you when there is a solution. I was playing Garrys mod last week and it was working just fine. I just got the half life complete pack and downloaded a couple of the games. After that Garrys mod hasent worked.

May we have your system specs?

I have a base HP laptop. 4 gigs of ram. But i had garrys mod working awhile ago.