Tabs disappeared after saving and going back into a document - not showing in version history

Hi all, 

I'm not sure what has happened. I was working in an excel spreadsheet that had multiple tabs within it and was saving as I was going along. I came out of it and then needed to go back into the spreadsheet a few hours later and when I have reopened only the first tab is showing and the others aren't. They are also not showing in version history and aren't hidden. Does anyone know why this has happened and how I can get them back?

Thanks,

Amy