If it is less than 2 years old and wasn't forcibly pulled out, it would be covered under warranty.
In general the motor has to be replaced (part 6055.100), in some cases if the shaft is still relatively tight when pressed in, if you clean the socket and shaft very well and get the dry, you can place a drop of super glue on the shaft and use a hammer and recessed punch or a press to seat it firmly. Give the glue a half hour or so to really set before installing the propeller or else it may end up glued as well.