If one hides all rows {columns} in a sheet, then the only way to unhide it is
through "Format->Rows {Columns}->Unhide" menu.
So as it confused me for a moment ("Hej! Where is my sheet? {clicks-clacks} How
to unhide it? Oh, shit, it must be in the menu...") it can confuse somebody also.

Possible solutions:
- to add some 'unhide' button on place of hided rows{columns} labels
- to prevent user from hiding all rows/columns
(really there are no any good reason to hide them all, so may be some dialog
with "You are going to hide all rows. If you do so you can only unhide it with
'Format->Rows->Unhide' menu.  [OK] [Cancel]" will be good enough)
- both things from above =)