I use a Linode server, and have had about a half-dozen domains running on it for the past couple years without issue. It's running Ubuntu 10.04 and Apache.
Today, for about 3 hours, all http requests were refused. I was able to ssh in without issue, and looked at the Apache error log and saw nothing interesting other than scores of 'the mysql server went away' errors.
The system is set up to ping me if the io rate gets very high, which does happen when the Drupal sites do a mysql backup, but that didn't happen today.
I also did a netstat -aon and verified that the listeners for :80 and :8080 were running (Varnish on :80), though some of the instances had TIME_WAIT instead of LISTEN.
A few minutes after rebooting everything was back to normal. What could be the cause of this?