how to troubleshoot ssh on ubuntu if putty is not connecting

apt-get install openssh-server

Check if sshd_config is available /etc/ssh/sshd_config

If available, please check if the port is not commented out.

sudo restart ssh

If you get the error, "Unable to connect to Upstart", restart ssh with the following:

sudo systemctl restart ssh

If you are still not able to connect enable it in firewall