A couple of times over the past year or so, the dashboard wouldn't open when I clicked on it.
I don't recall how it corrected itself last time it happened, but this time I decided to look up the problem. I found this on the Apple Support pages:
To open Dashboard:
From a trackpad, swipe right with three or four fingers. To close Dashboard, swipe left.
From your keyboard, press the Dashboard key, or press F12 (Fn-F12 on portable keyboards). To close Dashboard, press the key again.
I didn't know I had a dashboard key but now that I knew, I clicked it and the dashboard opened.
OK, that's interesting.
Then I noticed the Fn-F12 and suddenly it all became clear. I pressed Fn-F12 and then clicked the dashboard icon and it worked.
So when had I clicked Fn-F12? It's the keyboard function to open Firebug in Firefox.