I have a 2000 rx 300 lexus an iy won't go into gear it just slides up and down

The problem could be a linkage issue, failed Neutral Safety Switch, failed transmission pump, or stripped transmission pump shaft splines in the torque converter. When the splines strip out, the pump no longer turns and the car will not move. If the fluid is suddenly overfilled on the dipstick with the engine running, this is most likely the case. To verify the issue, a pressure test should be performed. Ensure that the ATF is pink, not brown or burnt smelling. The transmission fluid and filter should be changed every 30,000 miles.

If you are lucky, just a shift cable or link. However, it is 14 years old. How many miles?