You can try starting the ssh server:

sudo service ssh start

I tried to reboot my server, the run following command :
sudo service ssh start

I got : “ ssh stop/waiting”

I tried again with : “sudo service ssh restart”
I got : “stop : unknown instance ”

It looks like ssh failed to start, are there any errors in /var/log/syslog?

sudo tail -50 /var/log/syslog /var/log/auth.log

I got :
tail : option used in invalid context –5

There should be one dash only, not two, so replace --50 with -50. Does that work?

I tried both “–50” and “-50”.
And I got the same error messege.

Can you post the exact command that you’re running?

sudo tail -50 /var/log/syslog /var/log/auth.log
I got : tail : option used in invalid context –5

Then
sudo tail –50 /var/log/syslog /var/log/auth.log
tail : unrecognized option “–50”
try “tail –help” for more information

I tried :
sudo apt-get install openssh-server
sudo apt-get install openssh-client

/etc/init.d/ssh start

sudo service ssh status

It works, now everything is okay.

Thank you for helping me.