If you can still access the Settings menu on Apple TV, then do a reset as described in the related article: Reset or restore your Apple TV to its factory settings - Apple Support. This removes all user information and customization, but leaves system software as-is (or optionally updates it).
If you can’t access the Settings menu (e.g. startup won’t complete), then do a restore as described (8 steps) in: Restore your Apple TV - Apple Support. This would re-install system software (from a fresh download), and wipe user customization. Use a USB-C cable that support data transfer (not a charging cable as bundled with some other device). Power the Apple TV box at the indicated step (#4), not before.
For Macs with iTunes (macOS 10.14 or earlier) and PCs, find the Apple TV device icon in the iTunes application.
For Macs without iTunes (macOS 10.15 or later), find the Apple TV listed in the sidebar of a Finder window, in the Locations section.
If you can't restore your Apple TV, take your Apple TV to an Apple Retail Store or Apple Authorized Service Provider for more help.