To Fernando:
I had the same problem. I did not find how to get it back.
So, as far as I remember I simply flashed Android again (“dirty” install) (and maybe the BIOS aswell, not sure), and I got the boot menu again.
As I said, another way to fully empy the battery is to leave the tablet powered on on the BIOS menu untill the tablet stop.
You can access the BIOS by pressing “ESC” several time at the very begenning of the boot sequence (for this, you need to connect a physical keyboard on the USB port).