With the phone switched off press home, volume up and the power button all at the same time.
Do you then get into a special menu? If yes scroll down to the factory reset option and select that. This will reset your phone to an as new state. You may lose information stored on the phone but if you have things backed up you should be ok.
If you cant do this then the phone will need a repair.
How old is the phone? If its less than 14 days it can be returned and exchanged.
I would send it direct to Samsung as they have more scope on repairs
