Briefly unavailable for scheduled maintenance. Try again in a minute. This error message I got after upgrading themes in my WordPress. I did not worry too much because I always do backup of my website.
But instead of doing a file and database backup I found what could made a problem.
During upgrading themes WordPress created a temporary file located in the main html folder called .maintenance
Just simply delete the file and it should resolve the error message 🙂
Good luck!