Auto Headlights Not Working

Started my truck this morning to go to work, and as I drove off I noticed my headlights were off. I always leave them on "auto" and that function has always worked perfectly (even as recently as last night coming home). Once I switched selector to "on" lights worked normally.

My question is where do I begin to troubleshoot?

Where is the sensor located that detects whether it's dark enough outside to justify headlights?

Is there a fuse that could go out and disable the "auto" lights function, but leave the rest of the headlights' functions working?

 

The sensor is on the middle of the dash near the windshield behind the speaker.