Try starting it up in "Safe Mode" and see if it will fully load. Note: Not if all apps will load, as app access is restricted in "Safe Mode", just if the system will load completely. If it will start up then you have an app, virus or malware on the device that is causing the issue and you will need to remove it to resolve the 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.