depending on what windows you're using i would use a resque disk if you have one. or if you have an installation disk. reinstall windows. ( note if you are using windows 7. not to worry if you reinstall it. there will be a folder made in your program files call "old programs" all that was on your pc will still be there.)
Your electricity supply may be faulty or have been cut off. Your hardrive may be slightly damaged. Your graphics card may be damaged. The speed of your processor may be faulty. Some more help here: http://www.techyv.com/questions/computer-randomly-crashes-and-powers-no-display- signal