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My grandfather suddenly grew old and frail after my grandmother passed away out of the blue.
Literal
Grandfather [topic-は] grandmother [subject-が] suddenly-[mimetic-ぽっくり] pass-away-[after-てから] suddenly aged-up-[polite-ました].
A nicely layered sentence. The topic 祖父 (marked with は) governs the whole sentence, while the embedded temporal clause has its own subject 祖母 marked with が — the classic topic/subject split in subordinate clauses. ぽっくり is a 擬態語 for a sudden, peaceful death — 'without warning, just dropped dead.' It pairs conventionally with 逝く, the elevated 'pass away' verb written with 逝 (the same reading as 行く but with a 'depart for the next world' nuance). 老け込む compound-verbs 老ける ('grow old') with 込む ('into'), intensifying to 'grow old and frail rapidly.'