Have you tried doing a hard reset to restore the phone back to factory settings?
The procedure does and will delete all files and downloads that are on your phone.
How to do a factory reset:
1) Turn the phone off. If you can't, remove and reinstall the battery
2) With the phone still turned off, hold down the Power button+Volume up+Menu buttons at the same time for about 10 seconds.
3) Scroll down and select 'Wipe Data/Factory Reset'.
4) Select 'Yes – Delete All User Data'.
5) Select 'Reboot System Now'.