My Canon 300D is 12 years old and has worked perfectly until now. It will not switch on. Nothing lights up. There is no change to anything when the power button is switched on. Changing both batteries and CF card twice makes no difference. Does anyone have any idea what might be the problem?
Have you tried checking and/or cleaning the battery connections inside the camera? It might be as simple as one of the battery terminal contact springs having lost too much of its "springiness".
The battery and memory card doors on your camera have little prongs that press a recessed switch in the camera body to confirm when the doors are closed. If the camera thinks that either cover is open it won't power up. Have you checked to be sure the battery door cover and memory card door cover both still have the little prongs on them that fit into the holes containing the door switches on the camera body?
Other than that there's very little if anything else you can do that won't cost more than the camera is worth to check or fix.