tomber du ciel

To show up unexpectedly right when needed

Said of anything, or anyone, that arrives by pure luck exactly when it was needed.

Cette offre d'emploi est tombée du ciel juste au moment où j'en avais le plus besoin.

That job offer landed out of nowhere just when I needed it most.

The image seems to trace back to the Bible, to the story of manna raining down to feed the Israelites wandering the desert after their escape from Egypt. It's since drifted from the miraculous to anything simply arriving at a stroke of good luck, right when it's needed.