Try starting it up in "Safe Mode" and see if it will load. If it does, then you have something (malware, virus, bad app, etc.) on it that you need to removed. If not, then I would ask what hardiktechie did as well.
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.