Every SSH server has a (unique) host key associated with it, just like how you would use an SSH key on your end to log in to it. When you connect to a server for the first time, your SSH client saves the fingerprint of the host key and saves it. Every time you connect to the same hostname, SSH verifies that it sent the same key and is not another server pretending to be it.

When you recreated your server from a snapshot, a new host key was generated. Because they share the same hostname/IP address, you are getting a Host Key Verification Failed error while trying to connect via SSH. Since you know that that is the case, you can go ahead and delete the old host key fingerprint from your Mac so you can connect to your new Droplet:

ssh-keygen -R hostname

where hostname is the hostname that you SSH to, whether it be your domain name or Droplet’s IP address:

ssh user@hostname

As for the SFTP client error, No supported authentication methods available (server sent: publickey), it basically says that the server said it supported public key authentication only, and your SFTP client wasn’t able to provide that. Go into the client’s settings and add your SSH key to the authentication settings. It’s usually located in ~/.ssh/id_rsa.