Folks, can we be a bit more specific please? What are you trying to do, and what is happening and/or not happening?
First, the flash comes up automatically only in the "green box", Portrait, Close-up, and Night Portrait modes. In the Landscape, Sports, and Flash Off modes the flash is disabled. In the P, Tv, Av, M, and A-Dep modes you have to pop the flash up yourself by pressing the button.
If the flash is making a clattering noise and you get an ERR 05, then the camera is trying to pop the flash up. In this case there is usually something binding in the flash mechanism.
If you're in P, Tv, Av, M, or A-Dep mode (with the camera turned on!) and press the flash button, the flash should come up unless there's a flash gun mounted. As dithiolium and JWright have noted, there's a microswitch in the hotshoe that tells the camera that there is a flash gun mounted. If you get absolutely nothing—no sound, no action, no nothing—when you press the flash button, it could be that the microswitch is stuck.
Here's a picture of the switch on an XT—the switch is black on the XTi/400D:

IMAGE LINK: http://www.flickr.com/photos/dougpardee/133066175/
Poking around at the switch with a toothpick or similar can often free it up.