WordPress is certainly a very interesting system.
Especially for people like me who like to do things hands-on, it takes a bit of time and there is a lot to learn, but.. there are so many fun parts because you can create various things yourself.
However, since it is not a blog service provided by another party, unexpected outages also occur frequently. Over the course of more than a year of operating WordPress, I encountered a problem about three times, which was exactly
"This is the case where the site becomes unresponsive after updating a plugin."
I don't know the exact cause, but at this point, don't overthink it. Connect via FTP and delete the ".maintenance" file among the files in the HTML root directory.
Then, you should be able to see a properly functioning WordPress site.
Again, I don't know the reason..