Transmission

The simple answer is a bad transmission. To know exactly what the problem is means taking it apart. Do check your transmission fluid level just in case that is the problem.

Try putting in TransX or Lucas transmission additive. It's a long shot, but I've seen amazing things happen, if it's just a stuck valve inside. You'll be checking the level at the same time. Still dead, rebuilt trans.

If your car won't go forward but will reverse it's not your entire transmission something has gone bad related to the transmission you can replace it or you can find out what has gone bad first check the Automatic Transmission Forward Clutch Low and Reverse Band Lube Pipe is it going to be hard finding a mechanic to be honest with you about this because of how much money they can get find yourself a trustworthy mechanic if is going reverse your transmission is not bad if the car won't move at all then your transmission is bad and make sure your hand brake is off lol

Make sure the linkage from the shifter to the trans is not bad. Can you shake the shifter up and down too easily? If that's not it. put in Best Line drive train lube.(available on Amazon), or TransX or Lucas trans additive. Could just be a stuck valve inside.You can have the trans diagnosed by a shop before you spend the big bucks for a rebuild, if these things don't work. have you ever changed the fluid/filter? (not just a flush).