By default, Dimensions will not show you an inactive customer record in the list of accounts.
In order to view inactive customer accounts:
Click on Debtors, Customers (ribbon menu) or Sales Ledger, Records, Customers (classic/ribbon menu)
Click on the search icon
Click on the Window View button on the Customers list window
Tick the Include Inactive checkbox
The inactive account will now show in the customer list and can be selected and edited.