If the phone doesn't allow for a "Factory Reset", which is meant to allow you to save some of your stuff on it when it is reset, then you may need to do a "Hard Reset". However that will wipe all data. NOTE: If the device was rooted, this won't work on it unless you unroot it first.
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 "Volume...