The two most common reasons for this situation are
The user did have access in the past, but that access has been removed.
The user has access to a different area of MX than you. Some customers operate multiple MX environments.
Each MX customer has their own rules regarding how MX accounts are granted and whether or not they are taken away. This is not up to the MX team and the fastest way to understand what has happened to an account is to speak to your customer.
A common reason customers may remove an account is due to inactivity, but some customers will do so if they believe that there is a preferred way for you to exchange data.
Again, this is not performed by the MX team and account enquiries (new or existing accounts) should be directed to your customer and the team who look after accounts will be able to help.