Hello and Welcome. My name is Michael.
Based on the current condition of the unit, It looks like the best and fastest possible way out is to HARD RESET your tablet to its defaults. Keep in mind that by doing so it will set you back as the way the tablet was when you first purchased it deleting all personal data including pictures and videos.
First of all, make sure the tablet is FULLY CHARGED before you begin:
If your Nexus appears to lock up during any of these steps, you can restart it by pressing and holding Power for a few seconds.
Please let me know if you require any additional help on this matter.