Hi,
You need to perform a factory reset.
Before you do this make sure that your phone is fully charged.
''Be aware if you perform a factory reset, all user applications and user data will be deleted. ''
How to perform a factory reset:
1. Turn the power off.
2. Press and hold the Power/Lock Key + VolumeDown Key .
3. When the Factory Hard Reset welcome screen appears release all of the keys.
4. Press the Power/Lock Key to continue or any other key to cancel.
5. Press the Power/Lock Key once more to confirm or any other key to cancel.
6. Your phone will perform a factory reset.