2006 Foreman 500 ES shifting problems!!

Hello everyone, first I would like to say what a great site you have here. Lots of great info. The problem I am having is as follows: The other day I was out riding and I stopped the quad for a little break. When I got back on and went to shift into gear nothing would happen, the indicator showed that I was in 1st gear and it even made that little move forward as if it went in gear but when I gave it throttle it wouldn't move. I could shift it up or down and it wouldn't move. Took it to my guy and he is looking at. He said he thought he heard of an issue with a nut slipping off of the front of the gear box holding the parts in. He asked if I was going down hill at the time, which I was, and he said that this is when it usually happens. Any info would be greatly appreciated. Sorry for the long post guys. Thanks again.