Sometimes Moodle gets stuck in maintenance mode and you'll see the message "This site is undergoing maintenance and is currently unavailable" despite your attempts to turn-off maintenance mode. When you put Moodle into maintenance mode it creates a file called maintenance.html in moodledata/1/maintenance.html (the site files folder). To fix this try the following:
- Check that the web server user has write permissions to the moodledata folder.
- Manually delete the maintenance.html file.