Cannot add a worksheet to my Excel spreadsheet

I haver all of a sudden lost the ability to add worksheets to my spreadsheet program.  I am using macros that both protect and unprotect the sheets that are currently there, but I want to add another sheet, and the plus sign at the bottom is grayed out as well as the 'Insert Line' line when you do Home/Insert.  I have looked and I cannot find the setting that controls the addition of worksheets.  Any suggestions???