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. First of all, take off the phone's back cover and remove the battery.
2. Wait a few seconds and reassemble the phone. Lastly, press the Power button to turn it on. Done!