If your Apple Watch won't charge, read on.
Your Apple Watch maybe eligible for a free repair
Earlier today, Apple announced it would be providing free repairs to the "small number of customers" whose Apple Watch has been affected by a charging issue. In a support article posted earlier today, Apple acknowledged the problem, which has seen some Series 5 and SE owners running watchOS 7.2 or 7.3 unable to charge their Watch after Power Reserve mode (which effectively disables most features) is turned on. If you're not sure what model your Watch is, you can find out here. To check your device's current watchOS version, simply access the Settings on your Watch, tap 'General', then 'About', then you should see the version number.
Apple released a minor but important 7.3.1 update to address this issue, but should it persist, affected users will be eligible for a free mail-in repair. To see if your Watch is working as expected, place it on your usual charger and leave it for up to 30 minutes. If it still isn't charging even after the update, contact Apple Support to arrange your repair.
Even if your Watch is functioning as normal, it's always a good idea to ensure you install updates as soon as they become available so your device is covered with the latest security patches and features.