Resetting your Windows Phone will return it to the same state it was in the first time you turned it on. All your personal content will be erased, and the phone will be restored to its factory settings.
If you decide to reset your phone, you can back up apps, text messages, photos, and more to your Microsoft account in the cloud beforehand.
Be careful. Resetting your phone will erase all content stored on your phone, including apps and games (along with app data and game progress), text messages, call history, music, photos, and more.
In the App list, tap Settings
Tap About, and then tap Reset your phone.
You'll receive two warnings. If you're absolutely sure you want to restore your phone to its factory settings, tap Yes, and then tap Yes again.
It might take a little while for the process to complete.
If your phone is unresponsive and you can't reach Settings
Press and hold the Volume down
When you feel the vibration, release the buttons, and then immediately press and hold the Volume down
Once the exclamation mark appears, press the following four buttons in this order: Volume up
If you lose your phone, it's possible to reset it remotely.
Article ID: 10666 - Last Review: 17 May 2016 - Revision: 19
ERROR: at System.Diagnostics.Process.Kill() at Microsoft.Support.SEOInfrastructureService.PhantomJS.PhantomJSRunner.WaitForExit(Process process, Int32 waitTime, StringBuilder dataBuilder, Boolean isTotalProcessTimeout)