Web server currently unavailable (error 503 Service Unavailable)

As of around 8pm Eastern Time, Sunday November 22nd, the web server for our project (www.confluence.org) - and presumably others - is down, returning an error:

503 Service Unavailable
No server is available to handle this request.

A post was merged into an existing topic: Vhosts_rw in wedged state on Thursday 19th for about an hour causing 503