Well, you're not helping at all. You need to be more specific and tell more information.
1) What Operating system are you using? What version of Media Player are you using?
2)Are you using windows media player?
3) Do you have a current version of anti-virus?
4)Do you have a current version of some type of spyware scan?
5)Do you update them weekly?
6)Were you able to watch videos prior to xx/xx date?
7)Did you download new software that might have corrupted or took the default place of media player?
These are the questions you need to ask yourself, as well as, have an answer, before anything can be figured out.