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.
Best Answer
After months of searching I found this non-obvious, but very easy workaround on Mac OS X Hints.
While the majority of the tab isn't clickable, the very top edge of the tab is. So just click on its top border.
Still not sure what the underlying problem is, but this much easier then closing and reopening the document.