It has sit so long that the fluid has drained completely out of the torque converter, and it has lost "prime"...meaning it just won't pick up any fluid. Check it like this....check the fluid level on the stick before starting it...make a note of exactly where the fluid level is. Then start it, and check the fluid level again. If the level does NOT go down, the pump is not pulling fluid up from the pan. If this is the case, you may need to remove the trans pan, remove the filter, pull the filter tube down out of the case, and replace the o-ring, then re-install the tube, making sure it is all the way up in the case. Then replace the filter(new one, preferably), pan, and fill with 6 quarts fluid. Start, and let it idle for a few minutes, then check the fluid level. If low, add until full. Then see if it will engage the gears. If not, then do the fluid level before/after starting check again.

If it still will not move, it's possible it has just sit so long the rubber seals inside the trans have given up the ghost, and just flat will not seal anymore.