An easy way is to turn on your phone while pressing firmly "green" phone button (which you use to dial a number), "star" button and "3" button. If any of thesebuttons are not pressed then it won't work. Hold all these button pressed and turn on the phone and keep these buttons pressed until you see "formatting" on your phone. This will reset your firmware and evrything is gonna be retored to factory settings.
Cheers