Reset your phone.
Important: Resetting your phone erases all personal content, including apps, and restores the factory settings.
1. Make sure your phone has enough battery power, charge your phone for at least 20 minutes.
2. Switch off your phone, and disconnect the charger.
3. Press and hold the volume down key, and connect the charger. An exclamation mark (!) is shown on the screen.
4. Press the keys in the following order: volume up, volume down, power, and volume down.
Lumia 950 XL users: Please note the physical key combinations differ slightly compared to other Lumias.
5. Wait for your phone to reset, during the reset there will be 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.
6. Go through the initial phone setup, if the date and time are shown, ensure the values are correct.
7. Disconnect the charger.