Is it possible that the pane has been frozen? View->Window->Unfreeze
If not, try the following:
Type Ctrl-G (to bring up the GoTo box)
Enter a cell that is hidden (e.g., A1)
Type Alt-O (format) R (row) U (unhide)
If this unhides the row in question, then
Type Ctrl-G
Enter a hidden cell
Shift-click on a visible cell below (or above) the hidden row
Type Alt-O, R, U to unhide the rows
The same procedure should work for columns when you type Alt-O, C, U