First off, If you are still under warranty (up to 1 year after purchase) do not attempt to disassemble your phantom or try any solution requiring disassembly if you do not have a strong knowledge on how these things work and you do not have decent soldering skills. Talk to DJI and get it repaired.
You were quite vague in your post, if the crash was in fact not your fault (phantom randomly falling out of the sky) DJI will fix whatever is wrong completely free. If you did hit something and it was your fault you could attempt some repairs yourself since DJI will charge you anyways if it is your fault.
So that leads to some questions:
What exactly was the crash? Did it just fall out if the sky or did you actually hit something?
Any errors reported in the app?
Does the motor spin freely by hand, feel any different than the other motors?
You mentioned you swapped the rotors, by that i assume you mean just putting on different propellers and not actually doing any internal work on the drone, correct?
If you try to start the motors, what does it do? If you give the motor a spin while the others are turned on, does it do anything?
If you look inside the motors, are the copper wires in the bad one a different color than the other motors? Does the motor smell burnt?
If there is nothing abnormal the answers to those questions, I would lean towards you having and ESC failure. Unless you have A LOT of soldering experience, don't even think about replacing an ESC, I am not even sure I have read about someone soldering in a new ESC on this forum. In the phantom 3 models and up, the ESC is built into the mainboard and can not be swapped out easily like it could be in every other RC vehicle.