Shashidar, it could be just about anything...you really should give us more information if you want to get more answers.
So you have tried a new screen and it doesn't work. I would start by going "bare bones". Disconnect everything from the logic board except the Battery and Dock. Connect an iTunes enabled computer to see if it is recognized. If the phone does boot, then add one flex at a time until you determine what's causing the problem.
If it is not recognized, then try a new Lightning Port to see if that helps. If it still doesn't boot, then you have a logic board issue. That will require board level troubleshooting and micro-soldering skills to repair. If you are capable of doing this, let me know and I will give you some ideas for troubleshooting (or take a look here: PMIC gets warm when charging)