。
She died of cancer.
Literal
She [topic-は] cancer [because-of-のせいで] died.
~のせいで is the 'negative cause' connector — '[bad outcome] because of X / due to X.' The せい part comes from the noun 所為 (せい), meaning 'fault, doing, blame,' which gives the construction its accusatory flavor: when you assign cause with せい, you're often pinning blame, and the result tends to be undesirable. The neutral counterpart is ~のおかげで ('thanks to X'), reserved for positive outcomes. Cancer is conventionally written either with the kanji 癌 or in kana as がん / ガン — the kana form sometimes appears in news writing to soften the visual weight of a heavy diagnosis.