Glad you were able to sort it out so quickly!
Just a reminder for everyone since you mentioned your 500/500 service: the Internet is not involved in Movie playback at all. The Internet connection is used to download movies, to download Movie Guide updates and software updates, and to upload diagnostic information. But, it's designed so that it will work fine if there is no Internet connection at all. We have lots of systems on ships at sea, for example, or in RVs or aircraft.
As long as the movie you're playing is either imported, on an optical disc, or was downloaded completely (so that the cryptographic authorization step happened at the very end and it's not dimmed out in the OSD), no further connection is required to play that movie.
So, if you ever have a problem like this, look first at factors inside the house -- a failing Ethernet switch, a cable that got crunched somehow, a change to the house network configuration, etc.