Why WordPress gets stuck here
When WordPress updates core, a plugin, or a theme, it creates a temporary file named .maintenance in the site's root directory and shows visitors a 'Briefly unavailable for scheduled maintenance' page while the update runs. Under normal conditions that file is deleted automatically within seconds, once the update finishes. If the update process is interrupted -- a timeout, a memory limit, a lost connection, or a fatal error partway through -- WordPress never gets to the step that removes the file, and the maintenance page stays up indefinitely.
This is a state left behind by an interrupted process, not evidence by itself of what caused the interruption. The underlying update may have completed, partially completed, or failed outright, and clearing the maintenance file doesn't tell you which. Treat it as the first thing to check, not the last.
Clearing it safely
The direct fix is deleting the .maintenance file from the WordPress root directory (the same folder as wp-config.php) over FTP, SFTP, or your host's file manager. This alone restores public access, since it's the file's presence that triggers the maintenance page, not a database flag. It's a low-risk action -- the file has no other purpose, so removing it can't itself damage anything.
Deleting the file only removes the symptom. Before assuming the site is healthy, check whether the plugin, theme, or core update that was running actually completed: compare the installed version against what was expected, and check the site for the specific errors an interrupted update tends to leave behind, like a plugin appearing 'active' but broken, or missing files if the interruption happened mid-copy.
Reducing how often it happens
Repeated maintenance-mode lockups usually point to a resource limit -- PHP memory or execution time -- being hit during larger updates, particularly on shared hosting with tight limits, or during a WordPress core update paired with a full-site plugin sweep. Running updates one component at a time, during lower-traffic periods, and confirming the site has adequate PHP memory (WP_MEMORY_LIMIT) reduces how often an update gets interrupted in the first place.
WPFixAgent's connector snapshots the affected plugin or theme before applying an update and verifies the result with a fresh scan afterward. If an update is interrupted, the snapshot means recovery doesn't depend on manually confirming file integrity -- the component can be restored to its last known-good state and the update retried under controlled conditions.