Recnetly while doing a photo

2 possibles...

Most likely is the auto focus sub mirror retaining pin has broken, not an easy fix but possible if your careful.
Lift the mirror and see if the small mirror behind stays hanging down or lifts up out of the way.
if it stays hanging the pin is broken.
Instructions on how to fix here..
http://www.lotek.nu/misc/EOS_300D_sub-mirror_repair/

the other option is a faulty shutter, Canon say it should be good for 50,000 shots but many fail early with either damage to the vains or broken plastic parts, though usually you will get an error 99, no picture at all or a bright patch part way up. The shutter itself is fairly cheap (around £30 / $60) but you need to take the camera down further than the instructions above to replace it..

If you do decide to attempt a repair YOU MUST TAKE THE BATTERIES OUT AND LEAVE THE CAMERA FOR SEVERAL DAYS FIRST.
The flash capacitor is big on these and if you get too close to the flash transformer its likely to arc, giving you a hell of a shock and trashing the camera. It needs this time to discharge.
You can see it in image 21&22 of the instructions and you do have to handle it.

Best of luck