With the activation of WordPress Maintenance Mode, the temporary .maintenance file (which you can also manually create; see earlier section) is created in the main folder. After a successful conclusion to the maintenance process and the end of the maintenance mode, this file is usually deleted. Occasionally, however, an update is not correctly concluded. For example, this can happen if WordPress is accidentally closed during the maintenance process, or if the update is aborted because the plugin takes unusually long to load. In this case, the file is not closed, leaving you stuck in WordPress Maintenance Mode.
This problem can be easily fixed by manually deleting the .maintenance file. The only prerequisite for this is that you have the ability to access your web project via FTP (file transfer protocol).