I found a fix for this problem. It happened to me yesterday. It's software, not hardware. Simply connect the Zune to your PC, open the Zune software (It usually automatically pops up), click Settings in the top right hand corner, then select Device. Erase all content on your Zune and it will reboot automatically. My pad started working as soon as I did this.
Remember, you can always copy everything from your Zune onto your PC library, so do that if you want to save anything.