I have two answers depending on whether you have your exposure on manual, or auto. With the dial on movie, press MENU and choose the two-dot movie tab. Is movie exposure manual or auto?

Manual: in manual mode, you need to actually set your shutter speed aperture, and ISO. (Jog wheel, Av+jog wheel and ISO button.) The answer is simply that you didn't choose a combination that gave you the correct exposure. Half press the shutter to focus and tell you what the exposure will be, or press the "*" button for an exposure measurement only.

Auto: in auto mode, it only sets the scene once at the start. If you start shooting while your camera is in the bag, or the lens cap is on, then when you bring it out into the light, it will be overexposed.

Ah yes, I didn't notice that you said 500D, not 550D. That's a T1i instead of a T2i. I think the manual exposure on movie was introduced for the T2i so you don't have it.

This means I'm a bit out of my depth because I don't have access to a T1i to play with. There is this 'dummies' explanation of how to take movies with your camera:
http://www.dummies.com/how-to/content/how-to-record-video-with-a-canon-eos-rebel-t1i500d.html

Ah, OK I've found it now; downloaded the manual for your camera - this doesn't apply to the T2i, I don't think. Exposure will normally be automatic, and updated during the clip. If you press the ISO button, you invoke AE-Lock for the period of time labelled "Metering timer" in the menu. You can cancel it early with a different button that I have no easy way to describe... it's a frame with five points in it. I think you must have invoked this by mistake?

For T2i users, the movie-mode AE-lock function is invoked with the "*" key, and cancelled with the key to the right of it.