。
All the leaves have fallen.
Literal
Tree-[possessive-の]-leaves [topic-は] all fell-[completely-しまった].
~てしまう here expresses completion with a faint note of regret — the leaves have fallen, finally and entirely, perhaps lamentably. みんな ('all, everyone') is originally a noun but is often used adverbially without its particle, as here. A simple but effective use of ~てしまう to mark a scene of autumn's end.