Welcome to the forums. Is the battery good and fully charged? A weak or bad battery can cause shifting problems.
When you turn the key on, does the gear position indicator flash? If so, count the flashes, it's indicating a trouble code.
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.