The firmware in the player may be corrupted. Download the 01.01.20 firmware and install it on the player. If that doesn’t help, then you have a hardware problem, and you should contact Sandisk to have your player replaced under warranty.
http://forums.sandisk.com/t5/Sansa-Clip-Zip/Sansa-Clip-Zip-Firmware-01-01-20/td-p/270703
