The way I read his post is: "I made my last ball and the 8 ball fell in by itself without being touched by any ball."
If that's the case, most above have it right: you put the 8 back where it was and play the next shot from where the cue ball ended up. If the 8 had instead fallen in on the next shot just after you had hit the cue ball to make the 8 in the corner and the cue ball had gone straight into the pocket, you would replace both balls as closely as possible to their original positions and play the shot again.
The rules are here:
http://www.wpa-pool.com/web/the_rules_of_play
Here is the pertinent rule:
1.7 Balls Settling
A ball may settle slightly after it appears to have stopped, possibly due to slight imperfections in the ball or the table. Unless this causes a ball to fall into a pocket, it is considered a normal hazard of play, and the ball will not be moved back. If a ball falls into a pocket as the result of such settling, it is restored as closely as possible to its original position. If a settling ball falls into a pocket during or just prior to a shot, and this has an effect on the shot, the referee will restore the position and the shot will be replayed. The shooter is not penalized for shooting while a ball is settling. See also 8.3 Ball Pocketed.