It's the mistake of the floor guy. And where were you when he was laying the new floor - you are still the "general manager" when you hire a contractor, any contractor to work in your home. Did you tell the floor guy to mark the valve location?

Try to have the floor guy fix it. If you haven't pay him in full yet, don't finish paying till he does that. Otherwise...

Try locating the pipe/valve with a metal detector, but to cut the floor in the right place you need to be 100% accurate.

When a sub contractor did it to me, after I alerted him about it, I always found a way to get even. Except, don't ask me to tell you what to do or say. Or you could pay extra to have it done.