Hopefully that's all that's gone wrong.. but it's pretty rare for that to happen, in fact usually the lock nut is cotter pinned in place precisely so what you suspect cannot occur.
I would think that the prop would occasionally "catch" and try to turn a bit even if it had slid down the taper enough to lose the keypiece. (but you'd have no drive - just some noise.
Hopefully the shaft is still in one piece!
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"