Weird! I've never had this problem before. Either it doesn't work at all, and I have to download new codec, either it works just fine. But it never happened to me that an episode who was working just fine didn't work anymore (except when I burn them on a CD - sometimes, there can be an error that wouldn't allow me to play them anymore).
But I agree with Darcy about your options.
You should try too to play the video with VLCplayer (that's a free player that works WITHOUT codec, so it usually works to play videos that doesn't work with other player). You can find VLCplayer here:
http://www.videolan.org/vlc/ Hope it'll help.
