Occasionally some of the ribbon tabs in Office/Word/Excel 2011 become unclickable so you can't choose that tab. It's usually the ones on the left in my experience like Home or "Header and Footer". The only way around it I have found to date is to close and reopen the document.
I'm putting this question up so I can share an answer I just found after months of searching (probably was missing the right search terms). Hopefully we can also track if/when Microsoft fixes it.