Hi Sara,
Firstly, colonies sometimes die ... it's sad, but don't beat yourself up about it.
In your location there should still be forage about - you can't be too far from heather and it's been a good year for heather this year so it's unlikely to be starvation. As it's only one of your hives then it's more likely to be something to do with this colony and CBPV is pretty rife in some areas - and Devon is one of them.
If all the bees are dead you need to seal this hive up to prevent robbing of any stores left in there by your healthy colony, you don't want to transfer any problems to your healthy colony.
As has been said, there will be lots of help on here to assist in a diagnosis but we do need a lot more information.
When did you last inspect, were they queenright and was there BIAS, were there sufficient stores in there, how big was the colony, where it come from, how long have you had it, have you accidentally done anything that could cause them distress, have you taken a honey crop - all things that would help. Starvation of a colony is obvious - there will be no stores in the hive and there will be dead bees with their heads in the empty cells and their backsides sticking out. Piles of bees dead on the floor is disease or a catastrophic incident ... no bees in the hive and they could have swarmed, several times if you haven't inspected, and what few were left just died - any signs of open queen cells ? Varroa can be the vector and will kill a colony if the levels become overwhelming - had you checked for the levels of mites ?
There's lots of things that can cause a dead out - photos may not be essential but information is ...
