My nephews' friend (real person not the imaginary one) came over earlier and updated their Wii U. My nephews freaked out and just called me. Can confirmed 5.5.1 is the latest firmware luckily. Almost gave me an heart attack since I saw the thread title earlier today, but don't bother to check.
F*** this little twat changing other people's possession without permission
OP must get the game update snag and by interruptedly turn off the Wii U in game, OP bricked his Wii U. Lesson: Once being stuck updating, don't turn the console off.