log on to microsoft account whihc is active n your mobile and from there you can unlock your device. if your device is offline then you need to perofrm ahrd reset by following key combinations:
:>Turn off your phone.
:>Hold volume-down.
:>Keep holding volume-down and press power button for about 3 seconds.
:>Let go of the power button, but keep holding the volume down until image/icon on the screen appears.