Last year I bought an old Wizard riding lawn mower. Its got 12hp, 465 cc, and 38 inch cut. It ran great until, I was mowing, hit a rock, and the mowing stopped there. The battery went dead, so Ive replaced that. Still nothing. Where should I start, to get it to start?

Hitting a good sized rock can all but ruin your engine. Try removing the spark plug and turn the engine by hand listening and feeling for any tightness or locking up. I've seen a few like your where the fly wheel sheared its key and was now way out of timing. If the engine turns free lets start looking at the starting system. Hitting a rock should not kill a batery, probably a coincidence. Try putting another battery cables directly to the starter so see if the starter will engage. If it engages then try it with your battery bypassing the solenoid and ignition switch. This can go in several directions so try this and come back with your results.