Try starting up the phone in "Safe Mode" and see if it loads. While most apps won't load in there, but if it does start up then you know there is something on the device causing the problem. Bad app, virus, malware, etc. that you will need to remove.
If it won't load, even in "Safe Mode" then you may have to either resort to a "Hard Reset" or take it to a local tech to have it looked at.
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.