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Then view tips for the most recent OS version(s):
Windows Phone 8.1 Or view tips for all Windows Phone versions
How to hard (factory) reset
Warning: This will remove ALL data on the phone.
To hard reset your phone, go to Start - Applications list - Settings. Then scroll to the bottom and choose About. Then press Reset your phone. Then press Yes.
If you can't do that, then some alternative methods are as follows:
1) First press and hold the Camera + Volume Down + Power buttons simultaneously.
Then release the Power button when the phone vibrates - but continue to hold down Camera and Volume Down.
Then release the camera button and Volume Down when the Format prompt is displayed on screen.
Then press Windows button twice to confirm.
2) Turn the phone off. Then press and hold the power key until it vibrates. Then release.
Press and hold the Volume Down button. If the exclamation mark is displayed, then let go of the button.
Then input the follow sequence of keys: Volume Up - Volume Down - Power - Volume Down.