The Show command will cause all rows or columns in a selected area to be visible on the display.

Select the area of the spreadsheet where rows are hidden. To select the entire spreadsheet, go to Edit > Select All in the pull-down menus, or use the Control-A keystroke combination.
To show all rows, select: Format > Row > Show.
Similarly, to show all columns, select: Format > Column > Show.
After the area is selected, Show also appears in the right-click menu, if only columns or rows are hidden.