Are you SURE you have enough flud in it? If so,and it's NOT the shifter linkage loose or broken,go to Auto-Zone or Wal-Mart and get SEA-FOAM transmission treatment.Put it in and while sitting there running the engine,go thru all the gears,again and again until it goes into drive or reverse. Then run it down the street and around for a few miles while it cuts the varnish out of your servos and valve body. This stuff works great! don-ohio (:^)

Hi, check that the cable is attached to the gear selector on the trans and does the shift quadrant move as it usually did.... not sticking or stiff in it's movement. Auto's are basically a hydraulic pump forcing oil in through valves and galleries in the valve body,,,, it could be that a gallery is blocked or that a valve has jammed shut..... check all electrical connectors for a proper fit..... do all the lights on the shift indicator light up as normal, no engine check light coming on ?