If you updated recently, AND your wifi was toggled off on the next reboot the deck can’t restart into SteamOS. There is a handful of workaround that can fix it in some cases but essentially the Deck requires you to have internet access to verify the installation and if it doesn’t it will just get stuck on this step indefinitely.
Workaround solutions involve tricking it into thinking it is on a known network, ethernet into the dock, etc. Then loading the previous image.
The actual solution is for Steam to skip this verification step unless wifi is on and connected. It has happened in the past and with this being portable device if you happen to be using your deck in a normal way, bringing it with you traveling for example you’re fucked until you get home.
People keep downvoting users reporting issues because people think Valve is above criticism but every forum is the issue is being reported for the past several days.