Try turning it off and then start it up in "Safe Mode" and see if it will load. If it does, while access will be limited in that mode, it will let you know that there is something on the device making it not run correctly. Bad app, virus, malware, etc. If it still won't start up even in "Safe Mode" then you may either need to do a "Hard Reset" or have the phone looked at.
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.