Why does my camera turn off by itself?
The camera turns off by itself once I stop using it, why does this keep occurring?
The Canon EOS 40D is a 10.1 MP digital single-lens reflex camera released in August of 2007.
Try the following:
If the camera turns on again, you're dealing with the auto power off feature. The camera will turn itself off to conserve battery power if you don't operate it for a set time period. Simply pressing the shutter button half way will turn the camera back on.
You can change the time-out period, or disable auto power off altogether, through the camera's menu, as follows:
The procedure is illustrated on page 42 of the (English language) manual.
If you don't have the manual, you can get it in pdf format here.