I am not talking about the fact that someone snatched your office fatigue awareness ribbon (which should be bland-colored for sure) from your desk. I am talking about the tabbed toolbar which appears in many Microsoft office applications. You can see some examples of Microsoft office ribbons here:

Word Ribbon

Excel Ribbon

You can toggle it on and off by pressing CTRL + F1.

So if it disappeared, just try this combination and you will get it back.