Sounds like you either downloaded something that is stopping the device from loading, which can happen, or you fried something when it overheated.
To test this, try starting up the phone in "Safe Mode" and see if it will start. If it does, then something on the phone is what is causing the problem. You will have to find and remove the problem file/app/virus/malware or do a "Hard Reset" and wipe everything. If, however, it won't start even in "Safe Mode", then it is likely a hardware problem.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.