Argh!

I was just finishing up a family photo session and my shutter wouldn't release.  The view screen showed an error message:   

So I did. Nothing.
And did, again.  Still nothing.
I checked the battery, changed lenses, turned it off and on . . .
and got nothing.   
It seemed that my camera was done for the evening whether I was or not.  Luckily we were just finishing up and not just starting.
We googled it and found one hopeful post that said the camera just needs to be cleaned.  Hopefully that's all it is!