iphone randomly died, cannot turn on, will not sync, power is full...

Hello,

My Iphone was working fine this morning. I went to class, checked my email, everything was fine. The next time I looked at it, it was completely dead.

The battery was about 3/4 full, so power/charging is not the issue. I cannot restart it, the screen is completely black. Holding the power button and the home button do nothing.

Plugging it into my computer to sync does nothing. My computer does not recognize that anything is even plugged in.

Does anyone know what is going on and how to fix this??

Thanks so much,
Sarah