Google Cloud Platform, unable to SSH into server. Asking for password

We edited a VM instance on our Google Cloud Platform to increase the size of the Boot Disk. We had connected keys generated from PuTTYgen to the server without any passphrases. Following the change in the Boot Disk, the PuTTY client reflects that the server is refusing our key, the browser SSH is unable to connect as well. After a while, PuTTY has begun reflecting that we need a passphrase for the key even though we have never set any passphrases at all.

Looking for experts who can help solve this issue or at least help recover MongoDB data present on the server on a highly urgent basis