Many reports of Microsoft Zune players (the 30GB model, thought some reports have also been coming in about the 80GB model) hanging on the boot at midnight Pacific Standard Time on 31st December 2008.
A possible solution has been posted on The Register:
Use a small screwdriver to pop off the plastic shielding where the zune cable plugs into and remove the two screws on either side of the plug. Next pop the cover off and locate the battery plug at the top left corner of the zune, using your screw driver, pop the cable connection half way off, and do this to the other plug on the right side. Wait 3 seconds, then push down the right connector and then the left battery connecter. Your zune should start up immediately. Press the backing of the zune down (make sure headphone jack is aligned) and put the screws and cover back in place.
UPDATE: Microsoft has posted the following solution:
Follow these steps:
1. Disconnect your Zune from USB and AC power sources.
2. Because the player is frozen, its battery will drain—this is good. Wait until the battery is empty and the screen goes black. If the battery was fully charged, this might take a couple of hours.
3. Wait until after noon GMT on January 1, 2009 (that's 7 a.m. Eastern or 4 a.m. Pacific time).
4. Connect your Zune to either a USB port on the back or your computer or to AC power using the Zune AC Adapter and let it charge.
Once the battery has sufficient power, the player should start normally. No other action is required—you can go back to using your Zune!