Unfortunately there's no way to do that. I've seen various people asking - though usually for the ability to purge old data.
If you don't have a backup of your extract from before your failed refresh I'm afraid you'll have to start from scratch.
Do you have any idea why it only got some of the data? If you think that looks like a Tableau issue then it might be worth talking to support. You certainly don't want to have that happen very often.
Thanks Richard. After a bit of digging I've found the problem was the VBA macro that fetched the data from SAP BW and fed it into Access. I've now modified it to report if it retrieves less data than I asked for, then I can fix it before it enters Access and subsequently Tableau. In this case I was able to retrieve the data that Tableau did have, so my extract is now intact.