Error 5 means that the flash is obstructed. It probably thinks the flash is still on. Try putting it on and taking it off a few times. If that doesn't work I (caveat emptor) would grab a small flathead jewlers screwdriver and wiggle those little rails underneath the main rails a few times. There's a little button underneath it that senses when a flash is on-camera, and it's probably just stuck down.
If that doesn't fix it, or the probablem keeps returning I (again, at your own risk) would pop that little plate off the top and give it a good clean with some compressed air and anything else that I see fit.
This is what I would do, but I've been pulling apart mechanical equipment since I knew what screwdriver was. I'm also willing to take the risk. If you're not mechanically inclined I'd skip the second half. Canon would recommend you send it in regardless.
