My c5 trans lost all power to my 1 and 2 gear is in 2wd and 4wd

My 83 model f150, 4wd model wont move forward in first and second, my reverse and 3 and 4th along with over drive still work, but in first and second, it will try for a second, and then give up, lossing all power, and the engine is brand new and the fluid and filter was just changed. Ive tried it in 4w high and low, along with 2w high. Can anyone help explain what happened.