Today Apple released watchOS 7.3.1 for all Apple watch Series 5 and SE users. The update fixes a problem that has recently been affecting charging on the previously mentioned watch models.
According to the updates release notes, this update has issued a fix for a problem that prevented some Apple Watch Series 5 and Apple Watch SE devices from charging after entering Power Reserve Mode. As it is an update to fix this issue, the update is only available for Series 5 and SE watch owners.
To update to this latest update, you can go to the Apple Watch app on your iPhone and go to General > Software Update. Alternatively you can go to this location directly in your Apple watches settings app. Your watch must have at least 50% battery and be charging.