I initiated 'do-release-upgrade' over ssh, and then my (client) network connection died.
How can I reconnect to the session which is performing this command?
You cannot. Your command will be sent a SIGHUP. Take a look at:
Your session was killed when you disconnected.
To be more specific: presuming you su-ed to root, then your su process was a child of your login process, and your 'do-release-upgrade' process was a child of that.
When you disconnected you have killed your login process, killing the related child processes unless one or other them has done the sort of backgrounding that an daemon init does. (OT, but for more information on the latter, you can read about daemon forking at Wikipedia.)
You probably should have used screen, or worked on the system console :-(
If screen is installed, the do-release-upgrade command automaticly starts a screen session. This session can be resumed in another ssh session.