Unclehaggz wrote:As NoPro says battery compartment switch, others to check is the hidden CR2016 battery in the side of the battery compartment, used for storing date/time etc, that maybe corroded/flat causing the issue, but a wild shot!. The Compact flash model has a switch under the CF slot check that if you have one, check the SD/CF card flap too, as that has a switch located on it!
I have fixed a 400D with the same issue and it was an onboard fuse blown!, mammoth job to get to (strip camera right down), then replace fuse-link.
You could try turning it on with battery compartment open and use a very small toothpick to press the compartment switch further than the lid itself as the switch may have a broken element, also try with the CF/SD card slot.
Other than the suggestions above consult a Canon dealer!, is it very old/new/warranty etc Google has some very good online resources for camera fixes, you could also try them!
Good Luck
UH
Thanks Unclehaggz, will look more into it!