Unfortunately this can be caused by many problems, so it's hard to pinpoint the exact cause. Has this happened in the past or is it a recent problem? Try these three solutions below, that's the best I can do via forums.

1. Update your flash player
2. Update your video drivers
3. Try a different web browser or update your web browser to the latest update if not done already

Hope this helped fix your problem, have a great day

Regards,
Brosch91