So long as you don’t forget to say the prayer associated with that missing bead, you should be fine. It’s not a sin, and you don’t even have to use rosary beads. I use my fingers. God gave us 10 of them for a reason, after all, and I don’t think that’s a coincidence that the number of Hail Marys per decade matches the number of fingers. And it’s not a sin to use your fingers even if you lose a finger (Okay, that’s dark enough. I think I’ll conclude my reply here. Hopefully that’s not flag-worthy).
God bless you in your prayer life!