Instruction I:

 

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.

Instruction II:

 

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.