Why will my riding lawn mower go into all gears but not move?

I have a '93 ranch king riding lawn mower. I was using it without issue took a break and when I went back out I can put the gears in forward, idle, and reverse but the mower doesn't move at all. My nephew put the lawn mower on end and power washed it. I am wondering if he may have loosened a screw or something easy like that. How do I check to see if it's something simple or something difficullt?

Nancy
June 2012