How to Fix or Get Rid of "Briefly Unavailable for Scheduled Maintenance" message in WordPress

Reason: During the update of a plugin or a theme, you refreshed the page, moved to another page, or the upgrade failed due to some other reason.

WordPress creates .maintenace file in the top-level directory before the start of the update process. And removes it if the upgrade is successful.

Solution: Delete the .maintenace file.