Have you tried just rebooting the phone? That may well fix it.
Should that not do it, try restarting the phone in "Safe Mode" and see if it works fine. If it does, then you have an app, malware or virus on the device that is preventing it from working when out of "Safe Mode". You would then need to figure out which it is and remove the problem.
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...