What a sorry sight! I've only ever seen photos of piles of bee corpses like that in pesticide poisoning incidents. It is worth freezing a decent sized sample of the
bees in case it is later neede for tracing pesticide poisoning. If the bee authorities in your locality take an interest in abuse of pesticides, then they might later find your sample and photos helpful.
However, with sudden death like that there is just the possibility it is starvation, but if, as you say, you had just inspected, you would no doubt have seen if starvation was imminent.
Also, look for signs of diarrhoea which could indicate Nosemosis.