Canon 40d-pressing the shutter seems to short out the camera.

I recommend that you remove both batteries for ten minutes to get a full reset. you spoke of the main battery. When you open the battery compartment, look at the lower right toward the camera with the LCD facing you.
It is a little hard to see, but will say CR2016 3V. Stick your fingernail under the lip and just pull it out.
As I said I left both batteries out for about 10 minutes.
You will have to re-set the date and time after your done.