wash it cold and lay if flat to dry, pushed together as much as it needs. If you did not kill the wool, it will do.
If it really would not come back, then you could always get those thin elastic yarns, thread it in from the inside and give the ribbing the propper tuck. But washing and laying it will most likely fix it.
Continental Knitter with passion, pretty busy and always onto something new.
I have been away from the forum for a bit, but I am still around!