Hi,
- What exactly happens when the issue occurs? Do you receive any error message?
- Was it working fine before?
- What is the file format of the files that you are trying to play?
Generally, this issue should be fixed by installing the video codec. I suggest you to try re-installing the video codec and check.
Reference:
http://windows.microsoft.com/en-US/windows-vista/Codecs-frequently-asked-questions
Also check the file format which you are trying to play whether it is supported by WMP11 or not.
Information about the Multimedia file types that Windows Media Player supports
What kinds of video files can I play in Windows Media Player?
You may also try reducing the hardware acceleration:
a. Click Start.
b. Click Control Panel, click Appearance and Personalization, click Personalization, click Display Settings, and then click Advanced Settings.
c. Click the Troubleshoot tab, and then click Change Settings.
Note: Change Settings will be disabled if the graphics card drivers do not support disabling hardware acceleration.
d. Move the Hardware Acceleration slider until it is one notch to the right of None. This is the basic acceleration setting.
e. Click OK twice, and then close the window.
Let us know if it helps!