With the little info provided, the most likely culprits would be an herbicide or some mechanical damage. Ants didn't kill it in two days. In a container a large enough ant's nest may (note the stress on "may") affect the roots, but you said the roots were fine.

tj

Unless a plant is sprayed with a plant poison it will not die suddenly, there will be signs and symptoms for a fairly long time before that plant succumbs. If the potting medium was that dry it most likely was that lack of moisture that caused the death of that mint. The ants had nothing to do with the plant dying except maybe, possibly, increasing the rate of drainage of the potting medium.

kimmq is kimmsr

when a plant dies.. and when it looks dead ... can be a long period apart ... especially in the cool season ... then all of a sudden .. the temps come up.. and the plant dries up ...

growing perennials in hanging pots... can be very challenging .. one might suggest you 'won' .. getting it to live two seasons ... rather than being bummed ...

when you tipped it out.. how much of the media did the roots fill ... if they looked good.. only on the edge of the pot... they could have been all dead in the center ... i might suspect media failure which lead to water management problems ... long before the ants .... cut open the gob.. and analyze the inside ...

ken

My mint died overnight. Certainly no chemicals and the basil next to it is fine. The weird potential culprit is my cat who occasionally uses this planter box as a litter box.