In a post this week, Apple noted that a "very small number" of Apple Watch Series 5 and Apple Watch SE models running watchOS 7.2 or 7.3 might have issues with their batteries not charging after they enter Power Reserve, which is a feature that shuts down your watch to save battery life. The only function that works while in Power Reserve is the time.
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To determine whether or not your watch is eligible for a free repair, Apple suggests placing the watch on the charger. If it doesn't charge within 30 minutes, contact Apple Support to set up a mail-in repair. Of course, your phone will be examined before service to make sure it's eligible.
Apple released watchOS 7.3.1, which will prevent this issue from happening to other Apple Watch Series 5 or Apple Watch SE devices. You should update your watch as soon as possible for the bug fix, which is a good thing to do anyway so you don't miss out on the latest security measures and updates.