Do as follow:
* Disconnect the charger. Press and hold the Power and Volume down keys simultaneously. Release the keys when the phone vibrates and then immediately press and hold the volume down key until the phone displays an exclamation mark (!) on the screen. Release the volume down key.
* Press the volume and power keys in the following order: volume up, volume down, power, and volume down.
* Wait for your phone to reset. During the reset you will see gears rotating on the screen for up to 5 minutes, after which the screen goes blank for ~30 seconds and then your phone will restart.
* Go through the initial phone setup. Make sure that you have the date and time set correctly. Select Settings > Time & language > Date & time (Lumia with Windows Phone 8: Settings > date+time).
If above methods are not working then check [https://guideusermanual.com/product-name-lumia-620-manual&po=672223&lang=English|Nokia Lumia 620 Manual] for help.