Hmm... Few questions:
1. System specs
2. is the monitor cables connected up fully? (power and video cables)
3. are you using on-board graphics or a graphic card for your video?
4. If you have a graphic card, is all the 6 and/or 8 electric port(s) on it plugged into the PSU (if there is any)
they look like this
5. Do you hear any beeps when you turn the computer?