Hard Reset your Windows Phone will return it to the same state it was the first time you bought the phone. 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. To learn more about how this works (i will posting it on my next post)
WARNING
Be careful Hard Reset 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.
Procedure to Hard Reset your Nokia x/lumia Phone
2. Press and hold the volume up key and the power key for about 1 second.
3. When the phone vibrates, release the power key, but continue holding the volume up key,
until the phone is fully booted up. This procedure will boot the phone in recovery mode.
4. Use the volume up/down keys to navigate through the menu and select the wipe data/factory
reset option.
5. Delete all user data and press Power key to Confirm.
6. After delete all user data select reboot system now and press Power key.
7. Now your Nokia X/Lumia Phone will reboot.
8. Your windows phone is now ok.
Source: http://osasinfo.blogspot.com.ng/2015/06/how-to-hard-rest-nokia-x-and-some-lumia.html