Have you tried starting it up in "Safe Mode" to see if it will start completely then? Granted all apps won't run in that mode, but you can see if it will start up. If it will fully load in "Safe Mode" then you have something (bad app, malware, virus, etc.) on the device that is the cause of the problem.
If that doesn't do it, the device 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 "Safe Mode: OFF" rather than on.