Melly, first, I would put the battery on a charger and see what that does for the car starting. It is possible that with not running the car for a bit that the battery has just been drained a little bit and has a semi decent charge on it, but not enough to turn over the engine. Next, do a visual of the battery cables and make sure that they are looking good. If the battery was disconnected while doing the tranny change, the moving of the battery cables could have broken a cable inside the insulation and you now have a higher than normal resistance connection inside the cable and it is enough to allow the interior lights to light, but not enough to crank the engine. You can also verify this by keeping the car door open as you attempt to start the motor. If the lights don't dim at all, then it is normally a bad starter, if the lights dim slightly, then it is a weak battery, if the lights go almost all the way out, then bad cable.

If the cables tend to look good and there are no easy to bend spots in the wiring, then odds are you managed to have your starter fail at about the same time.

 

sounds like he used the wrong bolts to secure the starter,what happens people use longer bolts to secure the starter and in what happens is the bolts touch the torque converter not allowing the engine to turn over.

see if you can turn the motor over by hand if not you switched the starter bolts