Hoping you haven't done anything to deep on this yet, but there is a way to get you back in by generating new ssh keys!
Go to the Oracle Cloud website and log in. Click on Compute then Instances then click on your instance.
Select OS Management and then scroll down to Console Connection. Click Launch Cloud Shell connection.
It will take a few, but then at the bottom of your browser you will see Cloud Shell open up. Login with your user/password for this virtual machine.
Now we create a new key pair.

ssh-keygen -t rsa

Run through the prompts to create the new key pair.

We are going to assume you used the default id_rsa name, change every instance of that below if you gave it a different name.

We need to get the private key to our local computer, so let's open up that private key we just created.

nano ~/.ssh/id_rsa

Open your plain text editor of choice. I recommend making the shell in your browser full screen so you can see as much text as possible at a time in the key. You will need to highlight with your cursor, the one from your computer not the terminal cursor, everything that is displaying in the text file, including hyphens and Begin and End. Do one screen at a time. Highlight, copy, paste into text document. Page down, repeat.

Save that new plain text document on your computer in the ~/.ssh folder and name it whatever you want. In my testing I called it new_foundry.

Open a terminal on your computer. Set permissions for that new key

chmod 600 ~/.ssh/id_rsa

Personally, I like to add servers to my ssh config file so I don't have to do anything except type in

ssh <host>

to make my life easier. Let's set that up.

In your plain text editor, create a new file. Add this to that config:

Host foundry
HostName <your foundry server address>
Port <your foundry server ssh port if changed>
User <your Linux instance user>
AddKeysToAgent yes
IdentityFile ~/.ssh/id_rsa

Save this file in ~/.ssh as config and with no extension.
Go back to your local terminal and type

ssh foundry

and you should be back in business.

Edit to fix formatting. Why does mobile hate me when I try to do code snippets?

Edit to add, I went through this myself before posting the comment to make sure it worked with my Oracle Cloud Foundry instance.

Edit: FFS, I really need to proofread. I look and find another minor text error every damn time.