|Try two easy things first.|
1. Discharge the motherboard by turning the computer off, removing power, removing main battery too if it's a laptop. Before putting it back together hold the Power Off/On button down for at least 20 seconds.
2. The above only helps at odd times so assuming it is still the same, turn off the computer then start tapping F8 key continuously and hope that it shows the startup menu (list, white text on black). If it does then select Safe Mode with Networking. You can do a System Restore from there which doesn't lose your own data.
If you never ever see a thing on the screen after powering up then you almost certainly have a hardware fault. The first step would be to check with another monitor (or try an external if it's a laptop).
EDIT: Let us have the make and model number of the computer.
Always pop back and let us know the outcome - thanks
message edited by Derek