Well they are hidden rows not hidden columns so the advice on unhiding columns won't work. Presumably you've tried highlighting a row above to a row below and going to unhide.
Is it the case that the row height has been set to zero as suggested earlier?
You can find your data. Simply set up some more cells with the formula =(X)(x) where X is the column and x is the row number that is hidden.
If you open a new workbook, can you hide and unhide in there? If not, sounds like an Excel fault.
If you copy the entire worksheet to a new worksheet, formulas only, does it unhide the rows? Yes, that will work, I just tried it. Good Luck!!