A very small number of customers with Apple Watch Series 5 or Apple Watch SE models running watchOS 7.2 or 7.3 have experienced an issue with their watch not charging after it enters Power Reserve.
Find out how to identify your Apple Watch model. To see which version of watchOS you have, open the Watch app on your iPhone, or the Settings app on your Apple Watch. Tap General > About, then look for the version number.
What to do if your Apple Watch exhibits this issue
To check if your Apple Watch is affected by this issue, place your watch on the charger that you normally use, then wait at least 30 minutes.
If your Apple Watch still won't charge, contact Apple Support to set up a mail-in repair free of charge. Your watch will be examined before service to verify that it's eligible for free repair.
Apple has released watchOS 7.3.1, which prevents this issue from happening to other Apple Watch Series 5 or Apple Watch SE devices. Update your Apple Watch to get all the latest bug fixes and security enhancements.