what can i do if my iphone is overcharged?

The phone will not overcharge, as charging stops when the battery reaches 100%.

A reset may help. Tap and hold the Home button and the On/Off buttons for approximately 15-20 seconds, until the Apple logo reappears. When the logo appears, release both buttons.

No content is affected by this procedure.

As already stated, it is impossible to overcharge an iPhone. I leave mine plugged in overnight, every night, and I have for 7 years. No ill effects at all. There is something else wrong with your phone that has nothing to do with charging it for 5 hours. If stedman's instructions did not restart the phone there is a serious hardware problem. Are you sure you did it right?

Charge the phone for 15 minutes using the wall adapter (even if you don't see anything on the screen). Press and HOLD BOTH the HOME and SLEEP buttons AT THE SAME TIME until an Apple logo appears. This should take 10 to 15 seconds of holding. If nothing happens after 30 seconds of holding your hardware has failed.