Sounds like you may have no choice but to "Hard Reset" the device to clear the problem. If the phone will let you, I would save personal files (photos, videos, music, etc., not apps) to a computer or cloud account and then do the "Hard Reset".
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 Up" buttons, together, until you see...