You put the issue in the title, so it was cut off. I am going to take a stab at it and if I am missing something please reply here in this thread.
Try starting the phone in "Safe Mode" and see if it will start up. It will give you limited access, but if it does start, then you know there is something on the device that is the problem. Bad app, bad update, virus, malware, etc. You will have to find and remove the problem then before the phone can run normally.
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...