In case that you used a wrong firmware or the process stopped prematurely and the phone does not switch on, do the following:
Fully charge the battery and then remove it. Remove the sim and the memory card as well. Connect the USB cable to the phone and while holding down "*" insert the battery. Check control panel under Systems → Device manager (+) com ports to ensure that your phone has opened a com port. Start firmware flashing with the battery installed in the phone. The flash process will occur entirely using power from the phone's battery, therefore ensure the battery is fully charged otherwise the phone may crap out midway.