1. Check your cables, unplug and plug it back in again, this sometimes cures the problem.
2. Check you have up to date graphics card drivers from the manufacturers website. This may also cure the problem.
3. Go to device manager, accessible from control panel, system, hardware, device manager
Expand the display adapter and delete any drivers you find there.
Reboot and let windows reload display drivers for you. These will be generic plug and play.