If you are trying to update WordPress plugins and they keep spinning and appear not to update and you stop the update process then you may see the message Briefly unavailable for scheduled maintenance. Check back in a minute. on both the backend and frontend of your site. The first thing is not to panic as this can be fixed quickly and easily. Log into your site via FTP and in the root directory you should see a file called .maintenance Simply delete it and then the backend and frontend of your site will work again.
When you update any plugin then WordPress will briefly put itself in mainentance mode. If your server is very slow or the update times out then WordPress gets stuck and stays in maintenance mode. Luckily taking the action above will quickly resolve this!