Cell phones sometimes will develop an error that causes them to restart repeatedly and unprompted. Sometimes phones start doing this on their own, but sometimes it's after you've downloaded an app or received a file. The key to fixing the problem lies in figuring out the root cause, and then the solution is usually pretty straightforward.
Check to make sure the battery stays tight in its slot. One possible cause of a phone restarting on its own is a loose battery. The phone can turn off and back on quickly if the battery is slightly loose and momentarily becomes jarred free. If the battery seems too loose, you can fold a small swatch of paper a few times and place it under the cover to tighten the battery in place.
Charge the phone's battery to full power. You'll want it to have plenty of energy as you go about troubleshooting the problem. If the phone is restarting constantly, remove the battery and plug the phone into its power source, then replace the battery and do not turn it on until the charge has reached its maximum.
Perform a "soft reset" to set the phone back to its default settings. This process can differ for each phone but usually involves a tab called "Settings." where you can choose "Set to Default," "Reset to Factory Settings" or a similar option. Afterward, wait and observe whether the phone's behavior improves.
Delete any applications or other items you may have that were not obtained through sources officially approved by the phone manufacturer or your service provider. Even if there isn't a virus issue, an unusual file may cause trouble for your phone's software.
Download software updates to your phone if the manufacturer has released any. The phone manufacturer's website is the best source for these updates, and they frequently deal with glitches and other issues that may be causing the phone to spontaneously reboot.
Visit your phone's retailer or an approved repair shop to check for any other possible causes. An experienced technician might be able to solve the problem before you resort to buying a new phone or doing a full phone "reflash."
Reflash, or hard reset, the phone if nothing else will work. This means a process that completely deletes your media, contacts and all other data. This is typically a special process unique to each phone that you would normally only do if giving the phone to a different person. The user manual can provide the specific steps for your phone.
If the phone has begun continuously restarting on its own and you feel it's without provocation, then updating and reloading the software is most likely the best answer.
If the phone has been dropped or damaged, the problem may be due to a flawed mechanical connection inside the phone. This means it may not be fixable without the expert workmanship of a trained repairman.