On my main server after the most recent update server reset week of 8/7/2020 I noticed my main server with the majority of my characters showed that I had no characters.
After scrolling through the list I found the server in question Cairne in this situation and it showed no characters. I even opened a ticket and it was confirmed no characters on this server.
I went to the list of servers and selected Cairne and as soon as I did it went from no characters to all of my toons on this server. Posting this to help others who may be ion a similar state. Just locate the now empty server select it and it should repopulate the missing characters.
They connected Cairne and Perenolde with Cenarius during that reset.
Some of the recent realm connections on both EU and US servers didn’t go smoothly and caused some issues (guilds / gbank disappearing / duplicated items on the AH, etc) for some realms.
There was a sticky thread regarding the connections done on August 4th but it got unpinned after a certain amount of time.
I’ve seen posts from people talking about the same issue you’ve had.