You can use the DO’s Console Access on the control panel to login to your server.

First login to this page: https://cloud.digitalocean.com/droplets
And then go to your Droplet and click on the Console button on top right.
Using the console, you can access the Droplet using the root password without the SSH key.
From here, you can either configure your SSH server to allow login with passwords (I would recommend doing this) and setup the new SSH key or use the Console to setup the SSH key.

thank you that was very quick and on point answer!!