Cups won't start
Solution 1:
This happened to me too. Working consistently than rebooted to nothing. I found a couple of apparmor-denied messages in syslog but nothing productive.
Then I noticed /etc/cups/cupsd.conf had gone missing. That was the problem.
You could purse-and-install the cups-daemon but it does ship a default-level config file that you can just copy over.
sudo cp /usr/share/cups/cupsd.conf.default /etc/cups/cupsd.conf
sudo service cups restart