My computer wont turn on!??!?! HELP!?

I have a compaq preario desktop, (windows 7) and when i turned on the moniter today the screen was all black, i thought maybe i opened sim2 by acecident. it wouldnt go away so i turned of my computer by holding the power on. then i turned on my computer but it stays on the compaq screen. it suppose to show that screen for lke 5 seconds then go to another screen thta says Start windows normally, but its staying on that screen. i tried turning i off then on again but its not working. please help!