Transmission problem

My transmission went out I got a used one
from the junkyard it's been doing fine had it
for like 6 months drove to Mississippi like 2
weeks ago car did good on the ride today I
was driving stopped at a stop sign my car
wouldn't go engine reved so I try putting it in
reverse wouldn't go my friend pushed me
off the road to sit went to get a tow rope the  
car had been sitting for like 30 Mins I
cranked it up it drove just fine till I felt it
shift really hard into 3rd gear down a
backstreet someone pls help me out on
what it might be