Try the following steps:
This is factory reset, you should know what factory reset is.
Warning: This will delete all your data.
Make sure the charger is disconnected.
1. Switch the power Off.
2. Press and hold the Volume down and connect the charger, if successful an exclamation mark (!) is shown on the screen. Then input Keys in the following order:
1. Volume up
2. Volume down
4. Volume down