Nokia Lumia 800 - My Lumia (Windows Phone 7) does not work or respond as expected

Try these fixes if, for example, your phone freezes or does not switch on, if an app crashes, if you have sound problems, or if the screen is blank:

1. Restart your phone.

This does not reset your Microsoft Account (Windows Live) ID or remove any personal data.

  1. Switch your phone on, if possible, and disconnect the charger.
  2. Press and hold the Power and Volume down keys simultaneously until the phone vibrates. Release and the phone will restart. 

2. Reset your phone.

Warning: This erases all personal content on your phone and restores the factory settings.

  1. Make sure the charger is disconnected, and switch the phone off.
  2. Press and hold the volume down and camera keys.
  3. Press and hold the power key.
  4. When the phone vibrates, release the power key, but continue holding the volume down and camera keys, until the phone reboots.