Your haven't gotten any response here because apparently none of us can guess much from the information. It could be so many different things. Did you look it over carefully for signs of injury, skin lesions, etc.? It's possible that a tiny puncture wound or superficial injury became infected. It could have fallen and sustained internal injury against a rock or other hard surface. I do a "necropsy" on any animals that die of an unknown cause. If this should happen again, God forbid, look it over carefully with a magnifying glass, then cut the skin away, open it up through the belly and thorax with nail scissors and tweezers. Look at all of the organs for anything "different" such as clots of blood, foaming, tumors, visible parasites, greenish coloration, and even odor. You could go so far as sampling bits of lung, gut and stool if you know someone who works in a lab and has access to a microscope and some reference material. Other than that, it's just so hard to tell. A lot of us aren't anywhere near a herp vet, but that's an option if you have problems again.
That's a bummer for a first experience, but don't be discouraged. These little fellows can be pretty hardy. Just check out all of your practices, the tank conditions, temp. humidity, etc. Make sure it's clean and there are no "no-no's" in there. If you are going to replace your lost frogs, keep them quaratined until you are certain your current female is still O.K.
What sort of set up do you have?
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Patty
Lost River, Idaho