Welcome to the forums. If it won't shift with the manual shifter, I would start by removing the front cover and inspecting the shift linkage. With the cover off, see if you can shift with the manual shifter. If you can, the problem might be a broken gear, one of the gears that the shift motor turns. Sometimes, one of the gears will break and bind everything up, so it won't even shift with the manual shifter. If it still won't shift after removing the cover, inspect the shift linkage good and if you can't find any problems with the linkage, the problem is likely inside the transmission itself.
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.