In the Silmarillion, Miriel is an elvish woman, the wife of Finwe, the first High King of the Noldor, and mother of Feanor. She was famous for her skill at weaving, having the most skilled hands of any of the Noldor. She has a brief but important role in the Silmarillion.
After giving birth to Feanor, she said she could not have any other children, because the life that went into him used up all of her reserves. She continued to weaken, and eventually went to the gardens of Lorien, where she died from grief. Her husband remarried, and had two more sons. It was in part his mother's death, and rivalry with his half brother, that made Feanor angry and willful towards the world.
The Silmarillion is in many ways the story of the downfall of the Noldor. As a result of Feanor's oath, and his evil actions in pursuing them, the Princes of the Noldor, their realms, and often all who come in contact with them, are destroyed, one by one. However, Miriel's death comes not only long before Feanor's oath and ensuing evils, but even before Melkor has been unchained and left to spread evil in Valmar. There seems to be no other malignant agency at work, and yet Miriel dies from grief. Not only does she die from grief, but she dies of grief right after giving birth to her son. She is an elf, meaning that she has no physical difficulties, and she is living in Aman, the Blessed Realm, at a time when she could have no needs unfulfilled. And yet she feels the fire go out of her, and falls into a sleep she doesn't awaken from. Of her two lines of dialog, her final sentence is revealing, showing that she perhaps has some idea of what is to come: "But hold me blameless in this, and in all that may come after." After her death, her body remained unwithered, and her husband would come to look upon it, calling her by her names. Eventually, however, he stopped coming, and instead found a new wife.
I can not find, within the Silmarillion itself, a reason why Miriel should die of grief. Her grief would have to be a grief intrinsic to the world, since she had no agencies that would infringe upon what should be a joyous time. There is a chance that this death, which is perhaps the point where the Noldor begin their troubles, comes from a sadness that is part of the world even outside of the many evil beings that take a more dramatic role in sowing woe in the Silmarillion.