P800 constant reboot

I did a silly thing by sending my P800 to an un-licensed phone repair shop to have it cleaned of dust. Now it has been returned and when I turn the phone on it powers up and gets to the main menu screen and after about 3-4secs it just switches off as if the battery has been taken out and then it restarts on a continuous cycle of the same thing . Is there any way of fixing this problem myself (i.e. Hard Reset), and if so, how? If there is no way of me doing it myself then where do I takt it to get it fixed?