Maybe you forgot your iPhone ($249 at Amazon Marketplace) passcode. Maybe a 3-year-old got ahold of your phone and locked you out for a gazillion minutes. Either way, your only hope of using your iPhone again soon might be to restore it.
Hopefully you've kept up with your iPhone backup reminders. If that's slipped (no judgment), you're probably getting "iPhone Not Backed Up" and "iCloud Storage full" messages on a daily basis.
Use this as your guide for restoring your iPhone back to the original settings.
Make sure your phone is backed up
If you're already locked out of your phone, this step may be too late -- skip to the recovery mode section. If you're taking precautions, however, make sure you back up to iCloud or iTunes ASAP. If you don't back up, you could lose all your photos and videos.
Read now: Back up to prepare for your new iPhone
Go into recovery mode
1. Connect your iPhone to your computer and open iTunes.
2. Force restart your iPhone.
For the iPhone 8 ($155 at Amazon) and newer: Press and release the volume up button and then press the volume down button. Next, press and hold the side button. You should see the recovery mode screen.
For the iPhone 7 ($190 at Amazon): Press and hold the side button and volume down button at the same time. The recovery mode screen will appear.
For older iPhones: Press and hold the home and top buttons at the same time until you see the recovery mode screen.
3. A screen will appear that says "There is a problem with the iPhone that requires it to be updated or restored." Select Restore.
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