X crash and unresponsive input devices during 9.10 to 10.04 upgrade

While upgrading a box from Ubuntu 9.10 to 10.04 using do-release-upgrade, X crashed followed by a dialog saying "Ubuntu is running in low graphics mode". The keyboard and mouse no longer respond to anything. Luckily, I was using tmux so the upgrade itself could continue in the background.

However, I'm not sure what to do next. There's no telling if the upgrade succeeded or asking questions (e.g. debconf or new configuration files). I could not find a listening ssh remotely.