Try this stuff
1) Update bios(if possible)http://www.tomshardware.com/forum/308518-30-update-bios
2) Clear Cmos, do this if you cant start or have messed with the bios (don’t forget to unplug the computer from the wall and let it sit for few minutes, if you don’t have a jumper than remove the battery make sure the computer is unplugged and let it for about10 minutes before putting the battery back in) http://www.youtube.com/watch?v=Pdp_L5IxaNI
3) Check your psuhttp://www.youtube.com/watch?v=5FWXgQSokF4
4) Make sure your computer is hooked up correctlyhttp://www.tomshardware.com/forum/261145-31-perform-steps-posting-post-boot-video-problems
5) Run memtest(if possible) http://www.memtest.org/
6) Run Burnintest (if possible) http://www.passmark.com/download/bit_download.htm
7) If all else fails then try a gpu, Ram, or psu from another computer
Hope this helped!
have you been messing with any settings?