64 Dart wont go into reverse

I just picked up this car, its a 273 gt car. Everything was working fine then tonight it wouldn't go into reverse. push the button and it wont engage. it wont even pop the neutral button out. all forward gears and neutral work, just not reverse. I took the cable off and all the buttons function properly. i can manually shift through the gears with just the cable but not reverse. I'm not too familiar with the cable shifted trans so any help would be appreciated.

Kevin