1. Turn off the Device, Unplug it
2. Press power, Volume up & volume down keys together
3. Go to Recovery mode by pressing the Volume down key
4. Select Recovery mode by pressing the power key
5. A small red triangle will appear
6. Holding the power key Tap the volume 'up' key once
7. you will get four options - choose the one to wipe cache partition
8. Wait with patience till you get the message wipe cache completed
9. once you get the wipe cache complete message, you can see 'reboot system now' option above - Select the same and press power key - phone will reboot and should be back to normal now.
If this doesnt work contact the manufacturer (Google)