How did Kushina Uzumaki die?

She died when Madara took the nine-tails that was sealed within her and used it to destroy Konoha, but then Minato stopped him and sealed the nine-tails in Naruto. Kushina's chakras were also sealed into Naruto for when Naruto decides to control the kyubi.

(Kushina's appearance and talks about what happened 16 years ago, Shippuuden Ch. 499-501)