Hello,
I had the same experience few hours ago with an up-to-date zentyal 2.1.
rebooted than Zentyal was missing.
sudo /etc/init.d/zentyal start
started Zentyal, mail filter failed along with the mail module and still no admin interface.
sudo dpkg-reconfigure zentyal
still nothing - not even an error !!

sudo /etc/init.d/zentyal apache restart
... yay! Zentyal admin interface is back

Everything was back to what it was (after restarting the mail service).
Marcus