Pull off your shifter boot... and look at the plastic ball joint/bearing. You might have to remove it for a proper inspection. I would bet you that it's cracked. Also check the shift linkage for slop/wear. That could be your problem too. Gently move the shifter as you look at the linkage rods. You could have a failed or failing part.
f20 linkage just to give you an idea on the setup.
The shift lever bearing/ball... it rotates and locks into the shift carrier. Sometimes it can work its way loose and not allow you to get into reverse.
One other note... when you say you can't shift into reverse... do you mean the shift lever doesn't go into the slot? Try holding the gear lever in the reverse slot... then with your other hand switch off the engine. IF the shifter notches into reverse... then its your clutch disc hanging up on the input shaft splines. You need a new clutch in this case. Can you put the car in reverse... and then start up the car and MOVE the car? That would suggest it's your clutch as well.
Dackel