When was the last time you signed in to the old account? If it has been more than a couple of years, eBay may have deleted the account to prevent it from being hijacked.
If you are using the same computer and browser, go to your ebay.ca sign-in page, clear your current username from the field, and click on the empty field; it is possible that your browser may retain the old username as saved form data.
If not, try signing in using the original email address you used to create the account as your username.
If you still have access to that email account, you should be able to use the "forgot password" feature to receive a new password for the account, if the account still exists.
If you used the same email address to create your new account, that means the old account is gone.
