Try this...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
Be careful what option you choose once in here. If you want to wipe the system or do a "Hard Reset" just remember you will lose all personal data.