The first time I saw “Briefly unavailable for scheduled maintenance” on my WordPress site I was terrified. I had attempted to update a plugin and it failed. And no matter how many times I refreshed the browser, the site wouldn’t come back up.
Now I know what the solution is: delete the .maintenance file on the server.
WPBeginner’s solution for WordPress’s “Briefly unavailable for scheduled maintenance”: https://www.wpbeginner.com/wp-tutorials/how-to-fix-briefly-unavailable-for-scheduled-maintenance-error-in-wordpress/