OK well in fastboot type the command "fastboot OEM unlock" that will bring up a screen on the tablet warning you about the terrible woes of unlocking your bootloader go ahead and if that succeeds then you will see an unlocked padlock on the Google screen you bootloader is now unlocked
if this process fails or hangs at the unlocking stage then you have a corrupted nand partition (bad) the internal memory of the tablet has become corrupted this is the source of your problems and a new motherboard is in order. If it succeeded then we are on to the next step download wugfresh nexus root toolkit use the unroot back to stock option be sure to tick soft bricked and this will automatically flash a new factory image onto the tablet
Programming is a race between engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning.