Have you tried starting it up in "Safe Mode"? If not, I would try that next and see if it will load. If it will, then you have something on there you need to remove. Bad app, virus, malware, etc. If it still won't start up then you can either try a "Hard Reset", which will wipe everything and reset the phone, or it may have 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...