I've been told to try this but it hasn't quite worked for me (yet)-- this just started happening to my Zen, too. Stick a pin in the reset slot-- if it hangs there, wait until Recovery Mode shows up. Or, hold the power button to the left until Recovery Mode shows up. Then choose Format All (you will lose all of the files on the Zen) and then select reboot. This will take a while and mine didn't work yet. But you can also select Clean Up or Reload Firmware. I think firmware also makes you lose all files, but not necessarily the Clean Up option (not sure on that one). Hopefully it will work. Good luck!
AHak, September 2008