Transmission does not go into gear after car has warmed up

After first starting the car it shifts fine. But after approximately 10 minutes or after the car reaches normal temperature, the car doesn't go into gear! It just completely disengages from the gear and just sits there and revs. It started after replacing the head-gasket. Had to tow it home and it hasn't left the driveway since. After doing research I've found that it could be a speed shaft sensor, transmission solenoid, or a couple other things. Any Information would help a lot!