How can I fix it

If it is not starting at all but still getting hot you probably have a short on a main power line. The problem will take microsoldering equipment and skills to repair. If there is not data on the phone that is important to you it might be a better option to look at replacing the phone. If there is important data the good news is it is usually pretty easy to track down a short on a main power line. If it is something simple like a bad capacitor it is often as simple as removing the bad cap and the device will boot again. That is not always the case and it can easily be something more complex. That is my opinion from the description you gave.