Computer won't start

Hello, I just recently purchased a Nvidia Gt720 for my OLD 2007 compaq Hq. And I even went to geek squad to put it in and they told me that they could try but it most likely not work with this type of computer since its outdated... So I went home busted down the computer installed the card MYSELF WITH NO KNOWLEDGE of what I was doing... Somehow someway... IT WORKED!!! Saved me 40$ for that 2 minute labor But anyways not that I took it apart AGAIN to clean everything from all the accumulating dust.. NOTHING IS WORKING.. I don't know what the problem is.. All I did was take a bunch of dust out and boom... Gone.. Like I started it up and. Nothing is working... SOMEONE PLEASEE TELL ME... EMAIL AT ***@*** Please I'd be really great full.. Il give even more details if you do