Try turning it on in "Safe Mode" and see if it will turn on. If it does, then you likely have a virus, malware, bad app, etc., on the phone that needs to be removed. If, however, it won't turn on even in "Safe Mode" then you are looking at either a "Hard Reset" or possibly a hardware issue.
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.