Re: Why Lumia 920 Wont start
Hello and welcome to Windows Central.
There could be a myriad of causes:
1. Battery problem - though you mentioned that you tried using another Lumia battery, not sure how you did that since the battery of the Lumia 920 is not easily removable; Solution: Have the battery replaced in a service center
2. Power button is no longer working - probably dust accumulated or the electronic contacts are worn down. Solution: Have a service center check and repair it.
3. Software problem - corrupted OS; Solution 1: try connecting your phone to your PC with the Lumia Software Recovery Tool, and attempt to re-install the OS. Solution 2 if solution 1 doesn't work: Service Center.
4. Hardware problem - if the internals of the phone have started to fail (for some reason), then the only solution is to have a service center replace the parts.
Of course, if you know how to perform what the service center technicians would be doing to repair your phone, then you could just look for the spare part online and then perform the surgery on your phone yourself... although having a service center do this for your is more recommended.