So sorry for your loss.
A few years ago I was watching two mares that I knew would foal that night (pre camera days), All was peaceful so I decided to go make myself a sandwich and a flask of coffee. Returned 20 minutes later with my supplies to find a mare that was not expected to foal, had foaled - filly dead in the bag. I still think of that night and say 'if only......'. The other two mares of course duly foaled one after the other that night, just as I thought they would. But their two healthy babies never made up for the loss of the little one I wasn't there for.
Hugs for your mare - I hope she is coping ok with her loss.
Anna