Did you try restarting (turning off and back on) the phone?
If that didn't fix it, then I would try starting it up in "Safe Mode" and see if it works in there. If it does, then you likely have something on the phone that is interfereing. Virus, malware, bad app, etc. Then you will need to find and remove the problem. Or, if it works fine in "Safe Mode" you can back up your personal stuff off of the phone (to a computer or cloud account) and then do a "Hard Reset" of the phone. NOTE: Apps cannot be backed up, only files, pictures, contacts, etc.
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.