Transmission won't shift from low

1994 GMC 2500 Diesel 4x4 with auto 3 speed plus overdrive. Just quit shifting out of low. No funny noises, runs well in low but just won't shift. Something electronic, maybe? Did they have all electronic switching on the trannys in '94? All help is always appreciated. Thanks.

Does your speedo work? It should have set a few codes if it doesn't shift

check your fuses on the side of your dash.....it could be a fuse or a shift silond or a speed sensor but i would check my fuses first........then i would check for codes...................tommie99