I downloaded a spreadsheet and my computer automatically opened it in Excel. I didn't even look at the file type; just added data to it for a couple of days while clicking the save button occasionally to ensure my data wasn't lost. I seriously added at least two days of research to it across several extra sheets. I just rebooted my computer and discovered all my work is gone. Only the main page still exists. From what I understand, CSV files are only one sheet; you cannot save multiple sheets, but I would have assumed Excel would have warned me before I closed it that most of my work wasn't saved or would have given me an error message. I never got any kind of notice anything was wrong.
Is there any chance those extra sheets are still floating around on my system somewhere that I might be able to retrieve them from?