Why has selecting a keyword with a mouse click in Lightroom5 suddenly stopped working?

I have been working with Lightroom5.3. My list of keywords had gotten long so I experimented with adding hierarchy. I seemed to work as advertised with dragging other keywords under the parent keyword.

I left to do some other tasks. When I resumed I could no longer highlight (or select) a keyword in the list of all the keywords, let alone drag it.

I am using Windows 8.