I have just had a nightmare 24 hours! On one of my sites I tried to upgrade W3TC to the latest version and I more or less immediately got an error "The service is unavailable". This message then appeared on every single WP page, even the login.
On the (Windows) server there was a file .maintenance. I deleted the file and the W3TC folder and all cache files and the site now works fine.
What has happened here? I'm not a happy chappy!