Comment 4 for bug 1436956

I hit this behavior too.
I upgraded 14.04 on test laptop to 15.04 and it worked ok , but after last update system missing /sbin/init and won't boot.
before rebooted I tried to issue init command and get message the /sbin/init provided by upstart (systemd was not refenced)
also searching for init like dpkg -S /sbin/init results in error that no package containing /sbin/init
after reboot system brokes, restored by just copy /sbin/init, reboot and links from install cd of 14.04
doing dpkg -l upstart does not list /sbin/init, shutdown etc so --reinstall upstart does not help
aparently upstart 1.13 is broken
and system no switched to systemd while release upgrade