I recall fixing that. The screen was disgusting on the model I fixed. It was GREASY!. Powered it off, sprayed some cleaner on a lint proof cleaning wipe and cleaned it as best I could. NEVER SPRAY ON THE SCREEN OR GET THE PHONE WET!!!
If that's not it, then factory reset the phone and re-test. This is often where folk tell you they don't backup. Sometimes that's how folk learn about backups.