Welcome to the forums. I'm assuming you already tried rocking the ATV back and forth. I would pull the front crankcase cover off and check the shift linkage. If nothing is wrong with the shift linkage, you will have to split the transmission to find the problem.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.