Canon cameras typically have an on/off button on top and it is slightly below the level of the surrounding area. You almost have to use a finger nail to press the button.
And you must keep the button pressed for a couple of seconds before the camera will turn on.
This is to prevent someone or something accidentally turning the camera on.
This is the first report on this forum of one turning-on by itself.
First check the button itself and see if moves down and back up, when it is pressed by using a finger nail.
If it is stuck in the down position, that would indicate that it is jammed.
You might try exercising the button up and down by using a wooden match stick and see if that will free-up the button.
If the button is free, then the switch elements themselves may be broken. That would call for a repair by a technician.
If the camera is still under warranty, such a repair should be covered by the warranty.
My digital camera (Canon A630) seems to be damaged. It turns on and off automatically even when left off.