1992 Nissan Stanza Transmission Problems

I have a 1992 Nissan Stanza4 cyl 118k mls it started wanting to not run in reverse or drive when at idle but was running fine otherwie and all gears were working then all of a sudden while driving at about 20 mph it just wouldn't go anymore it doesnt go into any gear at all the transmission fluid was a little low but when full it didn't change anything what could the Problem be and how do I fix it?