1996 Chevy Cavalier help

I have a 96 chevy caviler, that I beleive might have a trans problem. Awhile back I was driving back home from work and heard a loud pop(almost like a shot gun)then the car would only do 30miles an hour. The car has been sitting for about 2 months now and today, as iwas checking it out and driving. The car would get up to 45 mph but would not switch gears and the tach was at 3500. Is there something that has gone out or is the trans shot?Some people at the parts store have said there is a 50.00 part that needs to be replaced and will fix the problem. Is this possible? And what part is it?Last question do I need to look at replacing the trans?The motor and eveything else runs and looks good no problems there. Thanks for your time norman