Performing a factory reset should resolve this issue. Do this from within the device system settings or via recovery:
Recovery:
- Power off the device.
- When powered off, hold volume up+volume down+power.
- Use volume down to highlight recovery then press volume up to select.
- When you see the android with exclamation mark, press both volume up+volume down at the same time.
- Use volume down to select wipe/clear data and power to accept.
- Reboot.
Be sure to backup and sync your device beforehand as everything will be cleared (except the sd card).