What do you do if your phone keeps on restarting? What might be the problem?

I will let restarting it... :P

90% cases will be solved by following

Switch off the wifi connection and check. IF it is fine then create a backup and force restart(factory reset) else copy the files by connecting to pc.

10% will be the hardware issue.

Hi,

It's possible that system is corrupted. Try to factory reset the phone. If still restarting there's a possibility that there's a problem in it's hardware.