Try starting it up in "Safe Mode" and see if it will start (instructions below). If it will, there will be limited access, but then you will know that there is something on the device that is causing the issue. A bad app, virus, malware, etc. You can then locate and uninstall/remove the problem item(s). Then reboot the phone. If it still won't start, even in "Safe Mode" then you might need to try a "Hard Reset" or even have it repaired.
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.