Did you try restarting the phone (turning it off and back on)? Sometimes that will resolve such an issue. If not, then you can either do a "Hard Reset" of the device to try and resolve it or take it to a local tech. In either case, you really will need to back up any personal data, photos, videos, contacts, etc., to another device or google account. Or risk losing it all. Apps you can't back up.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is...