First try seeing if it will start up in "Safe Mode". If it will (this is a restricted mode) then you have something on the device (virus, malware, bad app or two) that needs to be removed/uninstalled. Once that is done, then restart the phone and see if it will fully load. If it still won't, then you may have to do a "Hard Reset". However, I would only use that as a last resort, as it will fully wipe the phone and start it over, so you would lose any and all personal data.
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.