Your server was rebooted. Users have complained that the server refuses secured connections. What is the mostly likely cause?

A.
The public keys have been corrupted on the server.

B.
The clients are not resolving the server name properly.

C.
sshd is not configured to start in the default runlevel.

D.
The users need to ssh-keygen.

Explanation:
Check if sshd is running
Upstart: check the file /etc/init/ssh.conf and contains these 2 lines:
start on filesystem
stop on runlevel S
SystemV: check if the file /etc/rc?.d/S??ssh exists for your default runlevel.