What did Marie Curie die of?

Her direct cause of death was aplastic anemia, a condition in which the bone marrow loses the ability to create blood cells. This was most likely (although it cannot be proven) caused by chronic radiation poisoning - at the time she did her experiments, the detrimental effects of X-rays and radioactivity on living tissue were not known.

https://en.wikipedia.org/wiki/Marie_Curie#Death

As a side note...

Still, after more than 100 years, much of Curie's personal effects including her clothes, furniture, cookbooks, and laboratory notes are still radioactive...

Regarded as national and scientific treasures, Curie's laboratory notebooks are stored in lead-lined boxes at France's Bibliotheque National in Paris. While the library grants access to visitors to view Curie's manuscripts, all guests are expected to sign a liability waiver and wear protective gear as the items are contaminated with radium 226, which has a half life of about 1,600 years, according to Christian Science Monitor.

Her body is also radioactive and was therefore placed in a coffin lined with nearly an inch of lead.

The Curie's are buried in France's Panthéon, a mausoleum in Paris which contains the remains of distinguished French citizens - like philosophers Rousseau and Voltaire.

https://www.sciencealert.com/these-personal-effects-of-marie-curie-will-be-radioactive-for-another-1-500-years