Sounds like conflicting Display drivers.
Have you changed your video card recently?
There are various ways of correcting the problem, perhaps you should start by opening Control panel and in Hardware > Display > Properties.
Then select Remove and next shut down and reboot the computer.
With a bit of luck, when it reboots it will "Found New Hardware" and load the correct drivers for you.
If yop are still stuck tell us what set up you have (Video card, Monitor and operating version of Windows).