Briefly unavailable for scheduled maintenance. Check back in a minute.
Perhaps there’s a good chance that you’ve encountered this message in your WordPress site while updating. It keeps your installation from breaking while WordPress updates its stuff in the backend. There are times however that this message becomes the problem itself (which is a good chance why you ended up in this article).
What happens is that the scheduled maintenance doesn’t seem to be ending no matter how many minutes you check back. The message is stuck both front-end and back-end of the WordPress installation. This usually occurs when the updating process is somehow interrupted by abruptly cancelling or navigating to another page.
When I first had this problem I was pulling my hair off trying to figure out what to do. It was like the next worst thing to “white screen of death” (a colloquial term used by WordPress web developers when it displays nothing but a white, blank page). But then after scouring the net for a good 5 to 10 minutes madly looking for a solution I found out that it wasn’t so hard to fix.
Solution! (TL;DR people skip here)
When WordPress updates, it creates a temporary file in its root folder called .maintenance. It is then deleted automatically after updating. Sometimes WordPress fails to delete this thus you encounter this little error. You simply have to delete the file manually and surely your WordPress installation will be back to normal. Just check again the themes or plugins you attempted to update and try re-updating if it failed. (Just make sure your internet connection is good or you don’t do anything while it’s updating)