First off, have you tried resetting the device by pressing a holding the power button for 20-30 secs, releasing then pressing it again to start it normally?
Secondly, if this does not help, try leaving the Zip plugged in for a long time (pressing the REW button as described before). It may take several hours or even overnight for it to “wake up” and act right.
Do you have access to a Windows machine to see if you can successfully connect to that and re-load the firmware and/or format it? (Don’t forget the REW button hold while attempting to connect)
Will the player turn on when not connected to your Mac? If so, I would leave it on for a day or two to completely drain the battery. Then try connecting. Sometimes this is successful.
If none of this helps, I think you should probably call SanDisk Tech Support directly. they will probablty have you do some of the same things I’ve suggested, but if they cannot get it operating correctly for you, they can arrange a warranty replacement (provided it was bought new and is under warranty of course).