Try starting it up in "Safe Mode" and see if it will fully load. While you will have limited access, if it will load, then you know it is a software and not a hardware issue. I would first check the device for any malware, virus, etc. As well as any bad app or bad app updates. Once done, start it up again as normal. If that doesn't remove the issue, then you may want to go into "Safe Mode" again, save any files you want from the phone to a google account or a computer and then do a "Hard Reset" of the device.
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.