I have a creative zen

Try another PC or another USB port
Make sure the hold/lock is not engaged

Reset
Check for firmware update on Creative's website
Get Recovery Tool from creative

Try this: disconnect the player from the computer, turn it off, hold in the volume + button, and reconnect the player to the computer while holding the volume + button in. Keep holding the volume + button for an additional 5-50 seconds. Windows should eventually go into recovery mode and revive the player- it will pop up a window that says Found New Hardware Wizard.

If all else fails, check with this guy:
http://www.mycreativefansite.com/2007/05/myzenchecker-tools-guide-to-problem.html

Have fun!

-Tha Mp3 Doctor