Try manually resetting the BIOS by removing the battery from the motherboard. First, disconnect all power sources, including the monitor. Next, press the power button to discharge any residual power stored in your system (yes, it is true). Them, remove the battery, and wait 30 secs. Now, put the battery back and retrace your steps to reconnect power and the monitor. Power up.
If this fails. Try removing your video card and trying again.
Short of that, more advanced troubleshooting will be required. Will need to know your basic system specs and any changes you may have made recently.
Good luck!