When you go to View > Customer list, make sure the filters at the top are set to "All", then click on the header of the "Customer" column to arrange them in alphabetical order. Look for the "Cash Sale" customer. Do the same with the invoices, go to View > Invoices, set the filters at the top to "All", arrange the invoices alphabetically using the "Customer" column and look for the invoices of Cash Sale.
If the invoices or customer are not there, you can try to restore the backup folder, but that will replace all your current information with the one on the backup, there's no way to not affect your current data.
If further assistance is required, enter a support ticket at http://www.nch.com.au/support/supportcontact.html
Make sure to include your registration code and version of Express Invoice (v #.##) located at the bottom left corner of its main window.