Which account did you use when explaining the cheque? This same account will need to be used when explaining the bounced cheque (as well as using the same contact).
To explain the transaction with the same account, first head to Settings>Accounting>Account codes menu and tick the show in sales/purchases check box for the appropriate account and save at the bottom:
To explain the transaction with the same contact, head to Contact>Suppliers click on the name of the supplier and hit the edit contact button. Click on the Preset information tab and update the contact to a customer and supplier.