Try a hard reset on the phone. (Also known as a master reset)
A hard reset will erase all downloaded apps and user data. It will restore the phone to a factory default condition. It will be like when it was new.
Before performing a hard reset, ensure that the battery is fully charged.
Do not leave the charger connected when performing a hard reset.
Remove the external SD Card if any inserted.
Here is a link that shows how to do a hard reset. Scroll down to the section which describes how to perform a hard reset using the hardware keys.
Be patient, a reset takes a short while to complete.