Hi Kim,
I would suggest opening a support ticket for missing invoices as these are very specific to the account that they belong to and we would only be able to give general advice via the community forums. I would suggest, however, that if you are aware of any details of the invoice such as dates or which contact it belonged to that you could try and search within the invoice screen filtering by date or contact. If this does not work, please feel free to get in touch withy support for further help.
Kind regards,
Mohamed.