She may have abandoned it after the collapse...either because it scared her badly...or because she realized she was now exposed there. From my experiences watching pigeons and doves nest (have had several opportunities for both over the years here in Tucson)...they tend to build extremely temporary nest structures...lay an egg and raise the young very quickly, and then off they go to wherever it is they usually roost when not taking care of babies. That's why you rarely ever see "baby" pigeons...they grow VERY fast.