Black Screen On Boot Up

When I turn on my computer the screen just stays black,I hear the boot up sound but nothing else.So,here is how the story goes.I was playing a game and everything was fine until my screen has gone black but I could still hear the sound,but after few seconds sound stopped so I re started my computer and it didn't turn on,the screen just stayed black.I turned it off and when I turned it back on the same thing happened so I did than once more and it boots up like usual and because I saw that the game had some problems like that I didn't think nothing of it and just continued like normal.But I started playing another game and the same thing happened so I looked at some forums and they said it could be an overheating problem and because I haven't cleaned it very often I take the graphics card out and clean it and put it back in and now it won't show anything at all.And that's where I am right now.I tried re setting the motherboard,taking the graphics card out again and putting it back in and cleaning some other stuff,checking the cables but nothing.I think my graphics card died but I don't want to spend another 150$ just to check.My screen is working fine I have it hooked up to my laptop right now.
Specs:
Phenom II 955 3.6 Ghz (overclocked)
ATI Radeon HD6870 Powercolor
6GB RAM (4+2)
If you need anything else just ask.
 
Oh,I am very sorry.I tried to take out the PSU and it turned out that one of the cables was unplugged but now it brings me to a screen like when windows is not installed is that something broken or do I just re install windows?
 
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