Unhiding all hidden rows at once

Is there a way to unhide all hidden rows at once?

For some reason a whole bunch of my rows are automatically hidden, and it will take forever to unhide them all one at a time (I'm looking for one in particular, but I'd like it if they were all visible).
Also, when I highlight the rows above and below, right click and select "unhide," nothing happens. The only way I seem to be able to unhide the rows is to enter the name of the row in the namebox, then select the option from cells formatting on the home menu.

Also, why were all of these rows hidden in the first place?
(I have about 450 rows, it seems like about 20% are hidden)

Click in at the intersection of the row and column headers (top left corner). This should highlight the entire sheet. Now hover your mouse over the row numbers until the icon turns into a horizontal line with and arrow pointing up and an arrow pointing down (this occurs between row numbers). Double-click. This should unhide the rows.

NBVC said:

Click in at the intersection of the row and column headers (top left corner). This should highlight the entire sheet. Now hover your mouse over the row numbers until the icon turns into a horizontal line with and arrow pointing up and an arrow pointing down (this occurs between row numbers). Double-click. This should unhide the rows.

Click to expand...

Thanks!