There are several causes why Windows Media Player stops playing videos: 1. The video file format is not compatible with WMP at all. You should install extra codecs or convert the video files to WMV in such case. 2. Your video file has been corrupted. Then the video cannot be opened by any media player including WMP. 3. The Windows Media Player is corrupted due to various factors. To fix it, reboot your PC, update the software, and reinstall WMP from Microsoft website.