Use your other hand to set your joints/hinges(?) in motion. Increase that over time but start with small practises. After you did that for like 10-15 minutes you can "very gently" also stretch you hands.
This should increase bloodflow and increases flexibility again. My father has this and he also got operation because of it.
Don't overdo the practicing, better you do it regularly every day for a little bit then too long at once.
Also keep your hand(s) warm, perhaps you can get for cheap fingerless woolen gloves or have someone to get you a pair.