When I plugged my iphone 3Gs into the wall with 40% battery remaining, the low battery logo (red battery) showed up on the screen. I thought this was weird but still left the phone charging. Three hours later, the same low battery logo is still on the screen and the phone can't be turned on. I tried pressing home and sleep/wake buttons to restart, but the same logo comes back. I also have tried plugging into the computer, but iTunes couldn't recognize the phone.

Is my phone dead? Is there anything I can do? Thanks in advance.