Tranny Help

today my 1996 gmc jimmy made me want to :banghead: because it would not shift into any gear the shifter worked like normal but i never had any vehicle act like this it was like i never moved the shift lever let me know what happened i am a shade tree mechanic so i have dealt with the older transmissions but not the electric style. HELP!!! any info would be useful. O by the way when i shut it off and restarted it the transmission worked like normal. but then it will slip going from first to second.