it was going so well!
ots rotting! What's happened? can it be saved?
Plants are often overcrowded in baskets, and don't always get good airflow as the plants on 'top' prevent light and air getting in when they get bigger and cascade over the sides. You've possibly put too many plants in to start with.
I'd just cut away anything dead.
Is there any drainage? Seen hanging baskets where people have lined them with polythene bags and not cut any holes. The water has nowhere to go, so roots rot and plants die.
Could it be odd chilly night , or just generally leaves at bottom of plants die of , just tidy them up , the rest looks healthy , here is mine upstairs struggling due to being in open weather .