I've owned an apple watch since series 1, and I tried all of the mechanisms that have worked for me in the past, including the items shown in your reply (because I'm really used to using them). None of them worked - the control centre refused to show.
Then I recalled that on my last update (7.0.1) which I applied earlier this week, it didn't do a restart, unlike pretty much ever update I've ever had on the watch. At the time I thought "Oooh, that's neat, they've finally figured out how to update without requiring a restart - clever!".
So I held down the side button for a few seconds and when it asked whether I wanted to close down or notify emergency services, I selected close down. I then waited 10 seconds, and then held the side button down for 5 seconds until the apple logo appeared and it went through its restart process. After the restart, control centre is once more available. So, two conclusions - 1) they're not actually clever enough to have figured out how to update without needing restart, they just forgot to include that step in the 7.0.1 upgrade process, and 2) when in doubt turn it off and turn it on again.