1. Possibility caused by one of the apps you downloaded (may expose to malware, spyware etc). Try start the phone in Safe Mode. If the phone does not restart in this mode then it is most likely caused by an app. Find out what app this is and uninstall it.
2. Try wipe cache partitions.
3. If first two methods not solved, try backup all your data and do factory reset.
4. Lastly, if problem still persist, then you should bring the phone to a service center and have it checked.
