Try starting up the device in "Safe Mode" and see if it will completely load. It would restrict what you can do while in that mode, but it will help you determine if something installed on the device is the problem. If it loads fully, then you need to look into if there is a bad app, bad update, virus or malware on the device and if you may need to do a "Hard Reset" of it or not.
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.