This just happened to me 12/27/22. My unmodded WiiU had an update, and froze at 90% complete. I force shut it off, and "bricked" my console. Only WiiU logo popped up, and nothing else. Wound up buying a switch because i thought i was screwed. (Have over 100 hours of Zelda: BOTW in the WiiU) BUT...I tried something, and IT WORKED, at least for my situation.
***Turn on console while holding down the A B X Y buttons. (Hold all 4 buttons down at the same time). Keep all buttons held down until something changes. Eventually, the screen changed to a message prompt, then allowed me to sign in like nothing ever happened. It works good as new, and my save files are all intact. It sounds too easy, but I swear that is all I did. Hope this helps somebody out there.
P.s. i unscrewed the battery on the actual console, and put it back in before I tried this. (Its the only screw visible on the console) I don't know if this had anything to do with it, but I figured I'd say everything I did.