Start the phone into recovery:
- Press Power and Volume up together for around 10 Seconds
- be sure that the little arrow point on recovery (select with volume up, enter with volume down)
- When the open android wit red triangel is shown press Power and Volume up together again for short time
- Select Wipe data/factory reset with volume keys, confirm with power
- After factory reset select reboot system now
All your data will be deleted but you can access the device again if this will work.