1. Remove the battery.
  2. Insert the power cable directly to the unit.
  3. Power the unit on (hold power button on the keypad for 2 seconds)
  4. If the unit start as normal, insert the battery to the unit again and leave it connected for 1 hour.
  5. After 1 hour disconnect the power cable and put it back on the charging cradle.

The unit should now work as normal. If not, you might have to replace the battery.

You can also check the battery reset solution.