Well Thanks for all the info. I guess I will guess.
Next time you might offer up a little more info like the media player you are using.
What kind of file you're trying to play. Stuff like that really helps us help you.
You might need codecs. Try this one. Like I said it's only a guess.
Download the K-Lite Codec Pack.
You could also try another Media Player.
CODECs list for Movies/Audio/ Alternate players / Video help forums/ video-mp3 fixers