WordPress is awesome until it isn’t. Knowing you (or not really knowing you – but knowing how most people operate), you probably did the automatic update with no backup.
Yep, it’s what most people do. You shouldn’t do what most people do. Always perform a backup before upgrading WordPress. Anyway, if you did the automatic update and got the “Briefly unavailable for scheduled maintenance” message, the fix is pretty simple. You need to delete a file “.maintenace” in your root web folder. The update puts this there until the update is complete.
If you are using Fireftp or something like it, you may not be able to see .maintenance. You have to enable viewing hidden files in the options to see it. Don’t worry, if you are getting that message, the file is there. Now, if the upgrade didn’t complete correctly, you may still have some issues.