Dota 2 doesn't launch for me when i click "Play" dota 2 opens and my screen goes black but then it automatically alt tabs out to my desktop and i can still hear the Valve loading song but when i try to click back into dota 2 it still shows my desktop, this didnt happen before when i first downloaded dota 2, i tried updating my video card, Verifying integrity of game cache, restarting my computer, installing windows updates
Dota 2 only works when i use my laptop with its original screen, but when i connect my screen to my 23" monitor this error only occurs.

Can someone please help me?

Computer Specs
Hp Pavlivion Dv7
4 gig ram
ATI 4650 Radeon
Intel Core 2 duo P8800 @ 2.66 Ghz 2.67 GHz
Windows 7 ultimate 64bit