Hi
1. Was it working earlier? If yes, do you remember making changes to your computer?
2. What happens when you connect your monitor to a different computer?
Method1:
Boot the computer in safe mode and see if the problem occurs. Follow the steps from the link to boot in safe mode: http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode
Method2:
If safe mode works, uninstall the display adapter drivers from device manager and turn back your computer normally.
a. Click "Start" button, type “devmgmt.msc” (without quotation marks) in the “Search” bar and press "Enter". Click “Continue” if necessary.
b. In Device Manager, double click to expand "Display adapters", right click on the display driver, click "Uninstall" and click "OK".
Then, restart your computer and Windows 7 will reinstall the display adapter automatically.
This could also be a hardware issue if the computer does not boot up in safe mode. You have to contact your computer manufacturer for further assistance.