1) Can tall phlox be dead-headed?
2) One of my tall phlox turned brown almost overnight. What happened & can I save the plant? The phlox around it appear healthy.

Can you see any insects? Critters? If it were mine, I would pull it and dispose of it to help prevent whatever is going on spreading to your other plants. Once pulled you can see the roots and that may help find out what's happening, too.

I have had this type of thing happen as well.i just cut that part down to the ground and keep a close eye on the other part. It usually works out fine.