Providing the sheet is unprotected (look at Tools~Protection) you can unhide
all the rows by clicking on the grey box to the left of column A and above
row 1.
This will select all cells on the sheet. Then select Format~Row~Unhide and
Format~Column~Unhide.
A possibility is that the rows aren't hidden, but have zero height. Having
selected the whole sheet, you could try Format/ Row/ AutoFit, or Format/
Row/ Height to some appropriate value (my default seems to be 12.75).