40D shuts down when shutter pressed

You may have a serious problem that will require professional repair, but it's also possible that the problem may be battery related. Make sure that you have the correct battery type installed, and that the batteries are fresh/fully charged. If the problem persists, you may have corrosion on the battery contacts inside that camera that is preventing full battery power from flowing to the camera. Remove the batteries and wipe the inside camera contacts firmly with a dry cloth (heavy corrosion may require cleaning with a wire brush, steel wool, or sand paper). Remove any residue that may have fallen into the battery compartment during cleaning, then wipe both ends of the batteries and reinstall them in the camera. This cleaning solves the problem about 90% of the time, and I hope it works for you.