Emailed workbook shows no worksheet tabs

I have an Excel 2007 workbook that's been used for 5 months or so now. Recently, people I email it to say they can only see one worksheet in the workbook when they open it, and are unable to switch to any other worksheets in the workbook (they can't even see the tabs). They are in the "normal" view, and I have no idea what else could be wrong. Did I mess something up with the protection? Any other ideas to check??? Thanks so much!