How to hard reset the Nokia Lumia 1020 and earlier Lumia windows phone devices.
If you are having problems with your phone, sometimes a soft reset will fix the problem. The Nokia Lumia 520/522/525, 620/625, 720, 810/820, and earlier Lumia devices (710, 610 and 510) arrive with a removable battery that makes it easy to perform a soft reset. To perform a soft reset: remove the back cover from your phone and remove the battery. Re-insert the battery and replace the back cover and restart your device.
An update to this article which includes the newest Lumia devices including Nokia Lumia 1520 / 1320 and the 2520 tablet can be found here.
However, the Nokia Lumia 1020, 920, 925 and 928 and earlier Lumia devices (800 and 900) feature a unibody design, which means the phone is sealed and you do not have access to the battery. To soft reset your phone, use the following procedure: [The Nokia Lumia 1020 features the same key layout to earlier Nokia Lumia devices and the soft reset is shown here on a Nokia Lumia 800.]
To perform a soft reset, hold down the Power and Volume Down keys at the same time until the phone vibrates three times. The phone will restart after a few seconds and the Nokia logo will appear on screen.
Sometimes a soft reset will not fix the problem and on these occasions you may need to hard reset your phone. Warning: Performing a hard reset will result in the loss of all data. Please remember to take regular backups of your date.
Step 1: Backup all your data
Your windows phone is automatically designed to create a backup of your contacts to your Microsoft account. Other backup facilities include SkyDrive, Google and others.
Step 2: If your windows phone allows access to your phone settings: select Settings > About > Reset your Phone. [All your personal content will be erased and your phone will reset to its factory settings.]
If the phone will not power on, follow the instructions below:
Step 3: Switch the phone off.
Your phone needs to be switched off before completing the following procedure.
Step 4: Three(3) button reset
In addition to the method above, you can hard reset your phone via a three key reset as shown above. Press and hold the Camera Key, Volume Down and Power keys at the same time. Wait for the phone to vibrate. When the phone vibrates, release the power key but continue to hold the volume down and camera keys until the phone reboots to the start screen.
Note: Nokia has issued an additional set of instructions should the above method not work to reset your phone.