All but 3 apps have disappeared from Apple TV!

Could it be that you have set up this unit with an Apple ID account

that is set to a country where Apple doesn’t provide video services?

And/or a country where Apple doesn’t sell Apple TV boxes?

Availability of Apple Media Services - Apple Support

Else try signing out for the account(s), then sign in again.

The older 3rd gens did not run native apps, so it required a connection to receive its content and only the computers and settings would remain when it lost connection.

The newer models have an App Store and run native apps, where they are installed and stored on an internal drive. I have also not seen any instance where they have disappeared. I assume you aren't running any beta software? Make sure it is running TVOS 16.4.1

The only other thing to try is to wire it using ethernet, if it were currently on wifi. Beyond that you may need to go to a local Apple Store so they can run diagnostics.

The intent of my reply was not to solve the issue, as this is not something that can currently be solved by a user on this forum.

Rather, it was to note the difference in how the Apple TV 3 and the current TVOS based models run, to state that this is the first instance of this type of report here and that you would need to go back to Apple to do investigation. Apple may not be aware because this is not impacting a large base of users at this time. You can try escalating it through phone support first and see what testing can be done. Otherwise, someone may need to see what in person diagnostics can shed a light on.

Good luck.