If you are unable to boot the unit and reach the login prompt, then the recovery mode can be triggered to reset the unit by following these steps:
Power off the unit (holding the power button 10 seconds if necessary).
Momentarily press the power button so the unit boots up. You should notice the green LED light next to the front camera. As soon as you see the Nokia logo, hold down the power button until the unit turns off (approximately 10 seconds). You have now successfully interrupted the boot process.
Repeat the above step once more.
Boot the unit for a third time without interrupting the boot process. You will notice the text 'Preparing Automatic Repair' followed by 'Diagnosing your PC'. You have now successfully triggered the recovery mode.
Follow the on-screen guidance.