It's possible to make a customer account inactive. If you have a customer account that you cannot find, it's possible their account has been made inactive.

To find an inactive account, go tot the Customers tab and click on the search bar to expand it. Then change the Active dropdown to Inactive. Once you press search, this should display all the accounts that are currently inactive.

To make an account active again, click the red cross button to turn it back into a green tick. The same logic applies for Staff accounts too.

Only users with the Superuser permission level can deactivate or reactivate accounts.