SSH into instance using private key or shell provided by the cloud service provider.
If you want to create a new user than run
$ sudo adduser username
$ sudo addpasswd username
Go to etc/ssh/ and edit sshd_config file.
$ cd /etc/ssh
$ sudo vi sshd_config
Set Following as:
PasswordAuthentication yes
If you also want root user to login with password (without private key) set:
PermitRootLogin yes
Restart SSH service:
sudo service ssh restart
Exit your current session and login using your new credentials.
Protip: If you cant remember your private key use NoIP to get a domain name that you can use to SSH.