Instruction I:
1. Hold down the Power button in order for the menu to show on screen.
2. Choose the Power off option.
3. After a couple of seconds, hold down the Power button again. Your phone will turn back on. Voila.
Instruction II:
1. Take off your phone's back cover and remove its battery.
2. After five seconds pass, put the battery and the cover back in its place and press Power button.