Welcome to the forums. Have you tried rocking the ATV back and forth while trying to shift gears? If so and it still won't shift, you either have a shift linkage problem or a tranny problem. More than likely, a shift linkage problem. If it was an electric shift problem, you would be able to shift it manually. I would pull the front crankcase cover off and inspect the shift linkage. If you don't have a service manual, you can download one in the schematics/manuals section of the forum.