I’ve had the same thing happening on all my servers running Ubuntu Focal with unattended-upgrades enabled.
The cause was that containerd was upgraded automatically by the apt-daily-upgrade.timer job. In my instance the upgrade was from version 1.3.3-0ubuntu2 to 1.3.3.0ubuntu2.1. Docker daemon was shut down and not restarted. I had to start it manually.
This seems to be a serious mistake but on Ubuntu’s side rather than Docker’s.
Edit: There are further reports about this issue in containerd’s bugtracker with reports from April and yesterday.