I've got two in triage that had good bee numbers six weeks ago then last week found them with queen and a couple hundred bees. Plenty stores. No robbing. Chilled, dead brood present. No AFB. Best I can tell the bees simply perished.
I think that higher mite counts in autumn compromised the production of winter bees. The mild winter meant that the bees worked more than they should. Then we had two weeks of very cold, very poor weather. The older bees died earlier than they should have, leaving too few to cover brood. I expected this might happen and had a plan in case it did.
Both colonies will survive. I added an empty clean comb, a frame of emerging brood including bees (from another hive) and a few frames of stores to a five frame NUC. I caged the queens for 24hrs since her kin would be in the minority. They each have several more frames of honey for when they need them. I won't feed as all the neighbouring orchards are all in blossom.
So their demise began back in January when I committed them to an oxalic/glycerine trial. These two didn't do well. I persevered as far as I dared then they got Bayvarol very late. By that stage their troubled prospects were inevitable. This season I may mess around with mite treatment strategies through Spring and summer. Autumn mite assessments will be much earlier and more decisive.
The lesson for any beginner that reads this far (sorry for the novel) is that the death of a colony is seldom quick. The warning signs often present many months earlier. The hope is that we notice and have a plan ready.