Got it! Ulodesk, I stumbled across this purely by accident, looking around for some setting that might be doing this to you, and at first when I found the right one I didn't even notice it had fixed the problem. Do this:

1) On the ribbon, go to Formulas.

2) In the fourth ribbon section, entitled "Formula Auditing", notice that option H, "Show Formulas", is highlighted.

3) Turn it back off.

When I mouseover that option, it says it's to "Display the formula in each cell instead of the resulting value". It also says the keyboard shortcut for this option is <Ctrl-`>; I expect you hit that keystroke accidentally without knowing what you did. At least, I know that several times I've accidentally hit keystroke sequences that did something unexpected, unwanted and ununderstood.

I've never noticed that Formula-Auditing section before; now that I'm looking, it seems to have some useful features. In fact, the only thing I've ever used in the whole Formulas bar is the Name Manager. Thanks for your question, Ulodesk; I foresee a fruitful exploration coming up.