Although i am not Canon i seem to remember there is a very small button near the hinge of the pop up flash of some of these models, just check if that is up or down to start with.
The built-in flash could not be raised. Turn the camera off and on again.
If the error persists, contact Canon service.
Before doing that, set the control dial to one of the advanced modes: P, Av, TV, M. Reset power and try again. The camera should not attempt to open the flash and should not show an error. Then press the flash button, close to the EOS label, above the lens release button. The flash should pop up, make sure you are not holding it down with your fingers. If it does not pop up, try gently lifting with a finger nail while pressing the button again. If it still does not pop up, and you want to use it, you will probably have to send it for service.
@CameraClicker: thank you CC, because when I followed your tip I finally succeeded in opening the flash, after trying to reset the camera software and carefully tinkering with the little switches on top. I don't need the built-in flash because a have a 580 EX-II Speedlite, but I couldn't stand the fact that the flasher wouldn't pop.
A surprising culprit appeared when it finally popped: a dead fly inside the edge of the lid, flattened by the flash when it had closed for the last time, a couple of weeks ago. It had turned into some mix of jelly and glue. Once cleaned the flash works perfectly again.
Bzzzzzzz . . . .