I dropped my Canon 7D. Now I can't take a photo without zooming the lens, and all the photos have dark edges on top left corner. Is there a way to determine what's been damaged, and how can I fix this?
Clearly, something is broken. This camera has interchangeable lenses, so the first thing you need to determine is whether the body is damaged or just the lens. Switching for a different lens and testing out the camera is the first thing there. And if you can try the lens on a different body (borrowed from a friend?) you can also confirm that separately.
After that, you really need to send the broken parts in for repair. Check on Canon's web site for repair options for your country; for the US, that's this page.
If you don't have access to another lens or another body, you can just send it all in.
If the edge is only on one corner, it sounds like a lens element is probably knocked out of alignment. I'd try another lens to confirm this and then send the lens in to the manufacturer for service. You aren't going to be able to properly adjust the position of the optics yourself and it is possible something is bent inside that would make it impossible to properly align anyway. If the lens is particularly cheap, it may work out to be cheaper just to buy a new lens though, so it might be worth calling them first to see if they know a general price range for fixing your lens.