Okay, it sounds as if you are willing to reinitialize the device from scratch. So try this, which may or may not work depending upon how Lenovo's bootloader works:
Connect the device over USB to a PC which has the Android adb and fastboot commands. Linux is simplest here, because no special drivers are required. Then do this:
Press and hold POWER+VOLUME_DOWN until you get a bootloader prompt.
On the PC, issue this command:
fastboot oem unlock
Does that work? If not, then you'll have to google how to "unlock the bootloader" on your specific device. Otherwise, continue:
If it doesn't reboot automatically now, then reboot the tablet from the PC:
fastboot reboot
Does it come up working just fine now? If not, continue on.
Download the appropriate "TWRP Recovery image" for your tablet, and install it:
Press and hold POWER+VOLUME_DOWN until you get a bootloader prompt.
On the PC, issue this command:
fastboot flash recovery NAME_OF_TWRP_RECOVERY_FILE.img
If that worked, you now have a fully hackable tablet, and you can install a new operating system from scratch onto it. Cyanogenmod 13 (Android 6.0) is an excellent choice, if available for your tablet.