The only thing I can think of is it somehow logged out of your Apple Account therefore not showing your music & movie library. Strange.
There is a way to keep your apps etc when you reset:
Settings > Users and Accounts > select your user > turn on One Home Screen
Then when you reset or set up a new Apple TV, As soon as you sign into your Apple Account during set up it will download all your apps again in the same position on the home screen as they were previously. You will have to log into them all again though.