First thing to try - restart.
Secondly, check your monitor refresh rate:
1. Right-click the desktop, click Properties, and then click Settings.
2. Click Advanced, and then click Monitor.
3. On the Refresh Frequency list, click the appropriate refresh rate (at least 60MHz), click Apply, and then click OK to test the new refresh rate.
4. If the test screen is visible, click Yes on the Monitor Settings dialog box. If the test screen is not visible, contact your hardware manufacturer to inquire about the availability of a fix for this issue.
5. Click OK, and then click OK to close Display Properties.