So sorry to hear that. Chickens can hide their symptoms until it's really too late to do anything about it, and then they can die very quickly, so don't worry that you didn't notice or couldn't do anything in time. It sounds like you did all you could have done - I know it's easy to feel responsible for everything that happens to the animals in your care, but it's by no means your fault.
It could have been egg-bound - it's hard to tell. The usual symptoms (apparently - I hasten to add - as I've never had experience of this in my own flock) is that the hen will act as if straining to lay an egg, fail, try again... or do poos like scrambled egg (egg has cracked inside and infection sets in). You could have a look at her vent if you feel up to it. If you want a definitive answer, you might want to take her to the vets for an autopsy. There is an advantage in knowing, because although there's no bringing her back, you'd find out if she died of a nasty infectious bug that could have affected your other chicken and be able to treat the live hen accordingly. However, if she is feeding and drinking and generally behaving as normal, I would tend to assume she's fine and whatever affected no.1 hen is not infectious.
I would keep an eye on no.2 hen and if she has no symptoms after a week or so, assume she's fine and get her a friend or two
.