。
She got up late.
Literal
She [topic-は] getting-up [nominalizing-のが] was-late.
起きるのが遅れた turns the verb 起きる ('to wake up, to get up') into a noun-like subject of 遅れる ('to be late, to lag') via the nominalizing の plus the subject-marker が. The pattern '[verbal phrase] のが [predicate]' is one of Japanese's standard nominalizations: '[the act of doing X] is [adjective/verb].' Compare 食べるのが好きだ ('I like eating'), 走るのが速い ('runs fast'), 歩くのが遅い ('walks slowly'). 遅れる differs from 遅い ('to be slow/late' as state) by emphasizing tardiness against an expected time.