The system software may need to be reinstalled. Unfortunately, for the 4K models, users can’t do this themselves without access to the menus.
First try a cold restart (disconnect from power for 30 sec or so), then try to have it start up again. If that doesn’t fix it, then assistance from Apple may be needed.
From support.apple.com/appletv/restore:
If you have an Apple TV 4K
If you see a warning symbol ⚠️ while trying to update the software on your Apple TV, or see a black screen on your TV and flashing indicator light on the front of your Apple TV, your Apple TV might need service. Get service.
So please contact Apple for “getting service” (not a repair!) for your Apple TV box. Good luck!