Bondo Siding? I don't think so.
I don't suppose you have any left over? If it's more than a few years old, it won't match anyway.
Siding tool, shaped like a hook, to unhook the top, then replace the piece. Easy if you've seen it done, hard to explain.
Probably broke by a rock flung from the lawnmower.....
edit: found a youtube video:
http://www.youtube.com/watch?v=zBIA6bLXOfs