Gmod won't load, I just get a black screen

I have already defragged the files, I have verified integrity, but, whenever I start gmod, all I get is a black screen and the busy mouse pointer. Help! EDIT; IT happens with all my source games

[editline]10th August 2012[/editline]

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EDIT: FIXED IT. Problem with all games, not just source games. Reinstalled steam and ran SCANdisk