We had a Nessus scan but Nessus configuration was not completed on tenable add-on on the splunk side. Hence we missed the scan and data was not onboarded to splunk now we want to get data back , so how should we get that data back?
Splunk has to pull data from somewhere, logs or API, if you missed the scan, then the data should reside somewhere in the Nessus system.
You would need to look at the inputs configuration and see if there's an option based on the inputs to collect historical data from the Nessus system. I would start by referring the the documentation and see if that can help you.
https://docs.tenable.com/integrations/Splunk/Content/Splunk2/CreateInput.htm