First, the anode rod -- the part as I understand it that fell in the tank -- is either aluminum or magnesium. The anode rod is mounted at the bottom of the tank, so if it fell inside the tank, it should be directly beneath the hole.
Second, I would not leave it in there. It will continue to act as an anode and break apart, and that's fine. But what is not fine is that it will bounce all over the place in the tank, and that's a problem, especially when the tank is empty. The inside of the tank is lined with a ceramic material glazed over steel. The ceramic material slowly breaks off through normal use, so a rod bouncing around will quickly accelerate that process, and then your tank will quickly start rusting after that. Rust leads to pinhole leaks and rusty water, both of which require a replacement to solve.
Sorry to say, you've got to go fishing.
One of these should do the trick:
http://www.harborfreight.com/24-inch...ool-94162.html
Dave