It appears that the Russians still aren't sure what caused the problem. Contact was lost last night, then regained at around 7:00AM EST. That contact failed after seven minutes, but then contact was regained again at approximately 8:00AM EST. At that point they communicated with the station for seventeen minutes, and all appeared normal.
The communications failure raised the possibility that the station's solar panels couldn't be realigned, causing a complete failure that could lead to an "unplanned descent" (ie spacecraft falls to Earth, landing who knows where). Mir is scheduled to be ditched into the Pacific come February.