While I don't think you need to go that far to fix this, I will include instructions on how to "Hard Reset" the phone as well.
First, have you tried just restarting the phone? (turning it off and back on). This may resolve your issue. Second, what 'warning' is it giving you exactly?
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the...