...and we're back
It was due to server maintenance scheduled by our hosting provider that ran about 11 hours longer than scheduled. We expected downtime of about three hours in the middle of the night, but things went awry and we had to wait for our hosting provider to fix things on their end. They ran into problems with the automated configuration of networking for a large number of servers (including ours) and had to fix it manually.
Again, sorry! Everything is back up and working now. We will continue to monitor for any post-maintenance hiccups.