Dude, that sucks. Now you have to do an autopsy.
Take it out of the pot and look at the root ball. Rootbound? Smelly? (Yeah you have to sniff it; it's okay, sometimes I taste my runoff to see how salty it is, you won't die from smelling dirt)
Generally when you have a plant with minor root rot you can bring it back to life by re-potting into EXTRA light soil and watering lightly with some serenade mixed in, and keeping it protected form direct light. But your pot is of a shape that is hard to transplant out of, because the sides curve back in, and I bet it only has ONE drain hole- fast-growing annuals need more aeration than that.