Discussion on:

Office challenge: What's the shortcut for unhiding columns and rows in Excel 2007?

Control - 0 (Zero not O) Hides
Control -Shift + ) Unhides

CTRL+SHIFT+) Unhides any hidden columns within the selection.

ctrl-9 hides rows
ctrl-0 hides columns
ctrl-sh-9 unhides rows
ctrl-sh-0 unhides columns
I really hope these haven't disappeared in later versions
Alternatively, you can select the whole column (ctrl-space) or row (sh-space, or just click the headers), right click and choose Hide or Unhide as appropriate

Select rows (or columns) surrounding the hidden (rows or columns). Then change the width (or height) of one of the visible rows (or columns) via click/drage on the index row (or column). All of the selected rows (or columns) are now visible with the new height (or width). Finally, use click/drag to restore the original size. -- Note, this requires the initial row (or column) selection to be applied to the entire region, not jus to the indivicual rows (or columns).

As the lady wrote...
Ctrl + 9 does hide rows and Ctrl + 0 does hide columns.
Ctrl-Shift + 9 does unhide rows, BUT Ctrl-Shift + 0 DOES NOT unhide columns!
Don't know why they changed it, but - short of writing a macro (which I would recommend only if you frequently use this feature) - the quickest way to unhide columns in Excel 2007 is to select the columns on either side of the hidden ones, click on Format and type U + l (lower case L).

Ctrl+Shift+0 still unHides columns in my 2007 Excel

They might not be hidden but be small width, or height. Changing the width as one commentator suggested covers that possibility... or they might be filtered out