Christmas Cactus dying?

I have had christmas and thanksgiving cactus live for decades. If you watered it and it is wilty, then you probably have root rot from overwatering. The flies are fungus gnats that live in wet organic potting media. Let the top of the soil dry out between watering. Check the roots, if they are mush, then start over. You can break off the sections of the stem and let them dry for a day. repot in well drained soil. Keep out of direct light until they root. They like to be pot bound, so do not repot until they are falling over. Fertilize once and a while during the growing season.