Try starting it up in "Safe Mode" and see if it will fully load. If it does, then you probably have a bad app, malware, virus, etc. on the phone that you will need to remove. If, however, it still won't start, even in "Safe Mode" then you may have to either reset the device or have it looked at for 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 "Safe Mode: OFF" rather...