I think the extreme cold has probably killed it but I'd leave it for a few months yet to see if it revives. One way to check is to, very carefully, scrape away a little of the bark with a finger nail, if it is green underneath the rosemary is still alive.

mine did this a few ears ago and sadly it was dead. Every summer/autumn I take cuttings just incase it happens again.

thanks for your replies i agree