My camera won't fire.

This is a more or less common issue with the 40D (and as far as I know 30D and 50D as well, because same shutter button unit).

The shutter button is most likely not working because of some kind of dirt in it. There are two possible solutions I know of. Both aim at the goal of cleaning the button with use of cleaning fluid suitable for electronics.

1st: Complicated but "safe", dissasmble and clean the button part by hand.

- you should be able to follow this instruction to get to the button unit: Remplacement du bouton de l'obturateur du Canon EOS 40D

(The instruction is not from me)

- clean the shutter

-- a) either by disassembling it completely and clean every single part of it

-- b) or by just soaking the button unit in cleaning fluid and pressing it several times

- let it dry and take it back together

2nd: Quick'n'Dirty but maybe "unsafe", dont disassamble, just pour the cleaning fluid into the battery compartment

- remove both batteries from the camera (the main battery as well as the little time-save-one

- prepare a towel or something onto your bench

- rest the camera so on the towel, that the shutter is on the lowest point

- pour cleaning flouid in, it should flow directly through the shutter button to the outside of the camera

- it should NOT flow through the top lcd (don't know if really a problem or not)

- press shutter several times

- let rest and dry

I personally tried version 1a) and it worked. But over time the shutter button got worse again and I would have had to do it again.

As cleaning fluid I used 100% Isopropanol ( engl. Isopropyl alcohol ?), which was available at a local DIY-electronics shop (Conrad, in Germany).

Good Luck! :)