Try starting it in "Safe Mode" and see if it will load. If it does, then there is something on the device that is the problem. Virus, malware, bad app, etc. If, however, it won't even start in "Safe Mode" then you are looking at either a "Hard Reset", which will wipe the device, or having it looked at for hardware issues.
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"...