Try seeing if you can launch the phone in "Safe Mode". If you can, there may be something on the phone that is restricting it. Bad app, malware, virus, etc. You would need to find and remove the problem. If that doesn't work, or you can't get into "Safe Mode" then you may well have to do a "Hard Reset" on the phone.
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...