This happened literally over night. The soil was moist, as it usually is. The main branch's leaves are dead and soft while the stem is alive and green. There is another part of the growth (second picture) that is unaffected, as is a new cutting (third picture ) What do you think happened to the main growth and branches? Was our sudden intense heat?

I grow a lot of abutilon and this happens to me occasionally .. I have yet to find the reason. What I end up doing is take cuttings to root from the part that looks ok, leaving about 2 inches of the stem (normally I root cuttings in soil from around the mother plant but not when this sort of thing happens - discard the soil to be on the safe side). I then remove the whole plant from the pot and spray off all the soil from around the roots and cut the sick stem off and part of the crown if necessary then remove as many roots left from the sick part it that I can identify. Once that is done I root prune the whole root ball to no less than the length of the stem above the ground you kept. I let the roots soak for about a half hour in half strength insecticide/fungicide and pot it up.

It usually takes a couple of weeks for new growth to start showing.

Thanks @Anne. It sure is strange as I have two other stems growing just fine in that soil, green and growing as ever before. Do you think the cause is soil borne? Sad because it was doing so well and had so many b-uds on it.

I don't have a clue! Comes out of nowhere. I had a whole Tennent's White do that a few days ago .. I'm keeping my fingers crossed I can save it.