If all that damage has just developed in the past 24-48 hours then the odds are it is Late Blight. It is the only thing that takes a plant down that badly that quickly and looks like that.

Grey Mold is another possibility but in that case the disease would have been showing symptoms for the past 7-14 days, long before it got to this point..

Since it hasn't been at all productive and if all that damaged foliage on the bottom has apparently been left on the plant for sometime (why?) I'd strongly recommend just pulling the plant in an effort to save the others. Bag it and dispose of it far from the garden - do not burn it.

But if it is Late Blight then the odds are the other plants nearby have already been affected. Heavy dose of fungicide for them and cross your fingers.

Dave