Apple Watch - Black screen won't start

My husband's Apple Watch seems to be completely dead - despite time on the charger - even feeling warm on it - it will not boot - black screen - no startup even holding the two buttons down for an extended time does nothing. Any ideas?

I've tried holding one button, the other button - both buttons - nothing wakes it up.

Going to the Apple store is a nightmare - 1 week wait if you try to book it online. It's about 9 months old - maybe still under hardware warranty, but not sure if it doesn't have any support coverage.