I'm so sorry to hear. It does just break your heart and you wonder what you could have done different. I do learn something each day but I never seem to be able to catch up. Are you going to buy some pullets? I was too afraid I'd so something not right to do chicks, so started with pullets and am doing pretty good, although I have lost 3 of my birds since '06 for various causes. Keeping the coop cleaner, keeping varmits out better, keeping them cooler in our hot Texas heat, feeding them as many greens as I can chop for them. Putting out frozen water bottles too. I just have 6 girls, 4 silkies and two Belgian Mille Fleurs, so they are pretty easy to keep an eye on.
Hope your hearts will heal to the point you can get another birdy. Do make sure any new bird is same size.