Try this...
1. Go into "Settings" and then go to "Applications".
2. In here you want to tap on "Menu".
3. Next go to "Show system application".
4. Go to "System Interface" and then go to "Memory".
5. In "Memory" you want to clear all data and cache.
NOTE: This will wipe the data on the device.
Another way to do it would be to do a "Hard Reset". (instructions below)
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 device.
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...