had that happen to me once when flying my oldtimer. was going to check its dead stick and when i was getting to altitude i here a "plink whaaaaaaaaa" as the prop launched well i got to check the dead stick a bit sooner than i expected.

I acually found the prop later but didnt find the spinner nut (one of those big weighted ones)

I put a new one on a made sure that sucker was tight this time

BTW this plane was sitting around for a few years before I got it and I should have retightened the nut.. oh well lesson learned.