Help - my Challah bread!

My first guess is that it might have overrisen. If the temperature of the room was warmer, the dough was warmer, the yeast was more active, you left it to rise too long, etc, these are all factors that could all have caused it to rise too much.

From my experience the best test of rising is the finger test. You can get a real idea if the dough has risen enough and if it's risen too much.

Take a floured finger and press it into the dough, maybe half a knuckle depth or a little less. If the dough springs back completely it;s not anyway near done. If it leaves a slowly refilling indentation, it's just right, if it collapses not only where you press but also around it, it;s risen too much. As it collapsed around your finger, so it will do later. There is not enough structure to hold up the rise. In that case nothing to do but flatten it out and reshape the loaf and it should be ok. (It will rise even quicker then so watch it).

I learned this in my earliest breadmaking experiences, over 40 years ago. Made several loaves of sweet christmas bread and they looked so great, rose so high, and then when i baked them some collapsed as yours did, and some remained high, but the inside was hollow and cavernous, and the bread was horrible and dry.

Time and height and temperature don;t tell you enough about the state of the dough. Some bread is ready when it's relatively low, and other bread when it;s very high. But if it collapses under your finger, it's ALWAYS risen too much.

I think that the bigger the loaf, the slower you have to let it rise (that is, cooler room), in the last rise. What i've noticed when i;ve been in a rush and tried to make a dough rise quickly and put it in an unlit oven with pilot, or other warm space, is that the outer part rises fast, but the inner part (that's not close to the warm air) rises more slowly. Then i find a space between the crust and the inner crumb because the outside part has risen and left a large space between it and the inner part. This will sometimes collapse on itself later.

When you defrost your completed loaf, press it down flat on a floured board, and then do the braiding all over again. Leave it in the cool house temp, and it should be ok. I'm no expert and have no professional training but have done lots of bread over the years.