A maintenance mode means to show a console to your website visitor that your website is under maintenance and it will be back online soon.
You can set maintenance mode on while you are updating your site, sometimes if there is a problem that occurs during the update, this maintenance mode will remain as it is and your website will be stuck on this mode.
In order to fix this issue, you just have to open your site root folder and identify the .mainatainance file and delete it, it is a temp file created to show a warning message to your visitors.
Usually .maintainance file is hidden so first, you have to unhide it,
NOTE: if your update is timed out, in that case, you have to update your WordPress manually.
