If you lost your private key, there is no way to recover it, however, you can still access your instances by assigning a new key to it
First, creste a new key (either importing or creating a new one directly)
Then, shut off your inaccessible instance
Now, create a snapshot of it
Using the snapshot as image, launch a new instance, and select the newly created key.
Now you can connect to the new instance as usual. The username remains the same as before.