It's hard to tell, Baggies. I would look carefully along the stem for a hole, frequently with some mushy green or black stuff mounded up near it. That would be the entry hole for a stem borer, Then look up the stem, shining a strong light through it, to find the wriggly culprit. You then slit the stem lengthwise and winkle it out, and bind up the stem with a soft, porous cloth. After the surgery, prune off up to half of the foliage above the scar, to give the plant time to heal, before it wilts permanently. If it's not too late, it will resprout vigorously, and bloom and bear melons again. You might have to heat the greenhouse, to give the new melons time to ripen, though.