The gear shift won't go into gear. It moves up and down, but won't lock into place.
It sounds like your shifter assembly may be broken. I would recommend having the shifter replaced by a professional. This would require removing the steering column and a few other trim piece in order to access the bolts that mount the shifter to the steering column. There is a piece inside the shifter assembly that is supposed to lock the shifter into place when a certain gear is selected. I believe this piece is what is broken. Once the repair has been completed, the technician should be able to show you what broke and why the shifter would no longer function properly.