Mine went sideways on me two nights ago because I was installing Bioshock 1. I wound up having to hold down the power button for about ten seconds to force it to shut off. (Just because the screen is off doesn’t mean it’s not running.) After that, I was able to single press the power button for I think about three seconds and it came back on.
When you do get yours to come back on, I would set up a SteamOS recovery image and flash that thing back to factory and start over without a card in the slot. Once you get logged in, updated, and make sure it’s working OK, then put the card back in and see about getting that thing mounted and operational. It might not be a bad idea to format it too and start over.
Frankly this is quite preposterous. It is incredibly valid to discuss any issues people might be having with their product in a subreddit devoted to exactly that device. I can’t imagine more valid discussion points or a better place to talk about it.
The fact that people asking about or reporting an issue troubles some people is a bit alarming. I sincerely hope that people don’t make a Steam Deck so much of their personal identity that they can’t tolerate any critical discussion of the product.