Try turning it off and then starting it up in "Safe mode". If it loads then, it will have limited access, but that will tell you the problem is software. So a bad app, malware, virus, etc. If, however, it still doesn't load in "Safe Mode" either, then you would have to try a full on "Hard Reset" or you have 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"...