I've never seen them turn that color before.

If the death was sudden (one or two days) then I would guess that it was some form of root rot or some animal (like a vole) ate the root mass underground.

Yeah, I was thinking vole also .
Galloping Horse, what did the roots look like when you pulled up the plant?

Thanks for your answers. The roots were still attached, so I doubt it's voles. When I've had them in the past, I'd lift up the plant and there would be no roots, so the stem just lifted off. I was thinking maybe a soil disease, but I can't figure out what kind.

But isn't that a phlox there to the left that looks green and healthy?

Yes. Some of my phlox is fine, some (not just the one pictured) isn't. It's a mystery. But I saw a shoot coming out of the ground this afternoon where this one was, so maybe it's not entirely dead yet! I'm wondering if the extreme heat (the 2 weeks of 100-ish weather in July) may have been too much for it. Again, most of the other things in the garden did fine, even with that heat, so I just don't know.