You will have to hard reset your phone.
Before you do this ensure that your battery is fully charged.
Warning : Hard reset will erase all your settings and downloaded data
To hard reset your phone
1 Remove battery, and put back in after 10 seconds.
2 Press Volume up and Home button in the same time.
3.Holding Volume and Home button press Power button for 3 seconds.
4.When you will see Recovery menu choose "wipe data/factory reset" using Volume buttons to navigate and Power button to accept
5. Next accept option "Yes -- delete all user data".
6. After that confirm option "reboot system now"