Thanks, Paul, but it's a company computer and I have no control of the installation. I can call the help desk when I need to (they're in shouting distance).

It happened only with one document. I noticed that when control+H did nothing, if I pressed control+F, the Find menu would come up, and sometimes then I could get the Replace menu with control+H.

I can't post the document because it's proprietary, but I'm suspecting that there was something corrupt in it.