Maybe only the knob connection to the shaft is gone.
I would suggest to free the knob and see what's under.
If the shaft is turning and clicking, all you have to do is reassembling the knob or if it's gone, buy a new one and install it.
If the shaft is moving freely, well then, I think it's something for a repairman.
Gloire a qui n'ayant pas d'ideal sacro-saint se borne a ne pas trop emmerder ses voisins (Brassens)