password change

We have a username, “checkoutintegration,” but we’re not sure which email is associated with it. We need to either change the email ID or find out which one is linked so that we can reset the password. We’re currently locked out. It was functioning until a few weeks ago, but has suddenly stopped. We’ve been using this account for many years, and it’s possible that the associated email belongs to someone who has left the company. Could you please assist us?