Without knowing size, age, condition before molting, there is no way to tell. She (or he...) could have been dehydrated a bit before molt, and wasn't strong enough to recover. EVEN provided them with damp sub and a full water dish before they molt does not mean you can make sure they drink and are fully hydrated.
Remember, in nature, they die when it is there time. If every single T we kept in captivity lived forever in a perfect environment, well, it's just against the odds. Some will die even with seemingly perfect care. We do not know enough about genetics, defects, diseases, etc, to be able to make sure every single one lives to adulthood and old age. Don't kick yourself, you might have done nothing wrong at all.
Sorry for your loss, but truly, it happens. I lost one of my favorites yesterday, my miracle T Morado. He just stopped eating, and nothing could make him start again.