I don't understand what you mean by 'keep doing the same thing'. Is it starting and restarting over and over on its own? Or is something else going on?
If it is doing the restarting over and over, I would suggest pulling the battery and letting it stay out for about 2 minutes. Then follow the information below to start the phone is "Safe Mode" if the phone will start up fine in "Safe Mode" then you have an app or other software on the device that is causing the problem. I would then suggest removing anything you recently added/installed or updated.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable safe mode, simply repeat the steps above, and you should see "Safe Mode: Off" rather than On.