is it still under warranty??
if yes go to service centre.
if not head for xda dev and search for your device they have many guides to solve your problem.
basic you can do is press volume down+power button+centre button for 10 sec till you get the logo and leave it. it should go into recovery mode if yes go to factory reset and restart your phone(be advised this will erase all your phone memory and will be back to factory settings)