Hi and welcome to the forum
How long has the tank been set up for?
Have you tested the water for ammonia, nitrite, nitrate & pH? If yes, what were the results in numbers?
How long have you had the lobster for?
Is the lobster fresh or salt water?
Have you added any medications to the tank in the last few weeks?
When was the last time you did a water change on the tank and how much did you change?
If the water quality is good and the crayfish has not been poisoned, the claw could be broken at the joint where it meets the body and it can be removed. If you can grab the damaged claw and hold it, hopefully the crayfish will drop the claw and you can remove the claw from the tank. Once the claw has been removed the crayfish will grow a new one next time it sheds its shell. The claws usually take 3 or 4 moults before they are back to full size.