Apple Watch stuck on Sleep Mode

My Apple Watch has gone from being fully functional and waking me up every day to being stuck in sleep mode with no alarm is going off in the morning.

5 days ago my watch kept vibrating through the night. It seemed to be randomly inputting a pass key which in turn locked out my watch for 2 then 3 hours. That morning, following forum advice, I updated from 10.3 to 10.4 and reset the watch.

The pass key function has worked fine since. However, each morning the alarm on my watch has not gone off and the phone remains in sleep mode. As suggested elsewhere in the forum I've deleted the sleep schedule and now reset the phone 3 times. This is not making a difference.

Has anyone experienced this? Is there a fix other than resetting - which is not working in this case? Any thoughts and experiences would be greatly received.

At what stage does the watch go back to Apple and how does the return/replacement work?