The fitness app on my apple watch is frozen, how do I unfreeze it. The same app on my iPhone works fine

Have you tried force closing it? How about a forced restart?

Push in the side button once, find the app and swipe left on it and tap the red X.

Then tap the side button once more to go back to your watch face.

Now hold in the side button and digital crown for 10 second until you see the Apple logo then release