Apple Watch Ultra Digital Crown

Force Restart your watch and see if that fixes it.

Press and hold BOTH the digital crown and elongated side power buttons at the SAME TIME. Keep holding both button until the screen goes black and keep holding until the Apple Logo appears, then let go. Sign back in when prompted.

Did you actually try what I suggested you do? If not, do so. If after holding both buttons for several seconds you don't see the Apple Logo appear, then you have a hardware issue and your watch should be seen at the Genius Bar at your closest Apple Store. But before you do that, do as I recommended, then see if the Forced Restart gets your crown working again.