I don't know, of course, what has caused this problem between you and your daughter-in-law but your son probably knows, and you may want to talk privately with him about this.
Be careful, however, to avoid making him think you're being critical or negative about his wife; that would only make the situation worse. Instead, make it clear that you care for her and sincerely want to have a good relationship with her. Be open to any suggestions or criticisms he might have (even if you don't agree with them at first), and don't be defensive or negative. The Bible says, "He who listens to a life-giving rebuke will be at home among the wise" (Proverbs 15:31).
At the same time, it's important to remember that when our children marry, our relationship with them will change. At one time, we were the most important person in their lives but this is no longer true. Jesus said, "For this reason a man will leave his father and mother and be united to his wife" (Matthew 19:5). Interfering in their marriage can be very harmful.
In addition, pray for your son and his wife, and ask God to help you be a spiritual encouragement to them. Eventually they will probably have children of their own, and now is the time to lay the foundation for being a good grandparent.