Accidentally disabled transformation keys? I was working in Edit mode, merging a series of vertices, when I accidentally hit an unintended key combination which I now, unfortunately, don't remember. Nothing obvious happened, so I carried on.

However, now I can't use the keyboard transformation commands R and S. The transformation handles are greyed out, and though the transformation values are reflected in the Rotate (or Scale) status panel, there is no effect on the object and no change in the Object Data panel.

What have I done? And how do I reverse it?

I've spent some time googling, but I'm lost in a huge mass of hotkey information, and not knowing just what I've done makes it difficult to form a useful query.
You probably inadvertently hit Alt+, (comma), this turns on Manipulate Center Points, also found near the middle of the 3D view footer.



This makes all transformations only manipulate the origins of the selected objects, so scaling and rotating only do anything if more than one object is selected.  To disable it just hit Alt+, again or click the button in the footer.