Whats wrong with gmod?

all of a sudden garrys mod (i bought it from steam) has stoped working when ever i start it up it loads then shuts itself off and gives me a "client error" or somthing like that the games i also have instaled on steam is hl2 hl2 eps 1 hld eps 2 hl2:dm hl2 lost coast portal tf2 tf2 beta css css beta and finaly DoD source is it the games that are causing gmod to crash or somthien else I NEEDS SOME YAHOO WISDOM