This happens to me when the physical machine reboots. My machines run Ubuntu, ranging from 12.04 to 16.04. I resolve it by restarting supervisor as a service.
sudo service supervisor stop
sudo service supervisor start
(This somehow works a lot better than simply using ‘restart’)
Obviously this is not an ideal fix if you depend on Supervisor to start other programs for you without needing to be restarted after every reboot. I am currently looking into systemd like others suggested.
Edit: If you are on Ubuntu 16.04, this answer might fix all your problem as it did mine. You should ‘enable’ systemd to start supervisord. https://unix.stackexchange.com/a/291098