Samsung U-420 Cell Phone

Question

My son has a Samsung U-420 cell phone. It was left on the charger for several days (he was away from home) and when he picked it up to use it the phone was in an endless loop of "reboot". He tried holding the off button but the phone would not power down. He pulled the battery out and reseated it, to no avail. What is wrong with this phone and what can we do?