Last night I did a routine update of a plugin on this site.
During the update I accidently clicked on a link (finger twitch combined with a very sensitive mouse!) which interrupted the process.
The site then got stuck in maintenance mode and I could neither view the site nor could I get into the Admin area. To all intents and purposes the site was dead in the water.
The fix is very simple – just FTP into the site and delete a file .maintenance from the site root.
To be on the safe side, I then manually uploaded the latest version of the plugin I had been updating, just in case it had become corrupted during the process.