Apple watch won't turn on. Just flashes

I have the same problem, and the forced restart does not work.

Actually https://support.apple.com/en-us/HT204640 links to https://support.apple.com/kb/HT212180 ("if your Apple Watch Series 5 or Apple Watch SE is in Power Reserve and won’t charge"), which has been archived at https://web.archive.org/web/20210331153815/https://support.apple.com/en-us/HT212180

I'm not sure if the Apple support still allows mail-ins after the warranties (globally?) expired.

First and last time to me. Planned obfuscation.

Please fully charge the Apple Watch and try again.

If it still doesn't work then press and hold the side button and the crown for 15 seconds / until the apple logo appears. Then release the buttons. This may restart the watch.