I bought a Canon Rebel XT off Craigslist and it came without a battery door (to my prior knowledge). When I charged the camera batteries and put them in the camera the next day, it wouldn’t turn on. Does the lack of a battery door affect the ability of the camera to turn on and off?
The door comes off actually when you connect the camera to the hand grip. So the seller misplaced the door (there is a place for it in the handgrip to "hide it") when you were sold the camera.
Canon can get you a few one for a few dollars. My niece went through the same thing and she was able to get it in about a week. Go to a local camera store that sells Canon cameras (a real camera store, not an electronic store) and they can order it for you.
It is spring loaded–you’d squeeze the little thingie and it pops back in place.