Put the camera in to the full auto mode. If the pictures are still dark, it is possible that the apature inside of the lens is not working. There is a micro-motor, gear assembly, iris leaf assembly, and a servo amp circuit that drives the apature. The servo amp is referenced from the uPC according to a routine that uses light and speed evaluation.
In actual fact, anything that has to do with the light measurement, lens apature control, or the main picture processing can make problems.
It is a common problem that people sometimes damage the lense from banging or dropping the camera, or it just fails.
You will most likely have to send the camera to Canon service to have it fixed.
Usually answered in minutes!