Depends on how "protected" it should be.
Here's some code: http://www.mvps.org/dmcritchie/excel/hidden.htm
Try right click on the Sheet tab, select View Code then Select: Visible: 0 - xlSheetHidden.
BUT BE CAREFUL!! If you select Visible: Very hidden you can not even see that there is a hidden sheet - which makes it a bit harder to bring it back (need VB-code).

I presume you already know how to password-protect a sheet...menu: Tools>Protection>...

Hope this gave you some new ideas
Ola Sandström