What happens if the serve hits the net?

Answer 1

That would depend on whether the ball winds up landing in the service box or not. If the served ball hits the net and lands outside the service box, a fault is called. If the served ball hits the net and lands within the service box, a let is called.

Answer 2

If the ball lands in the service box, then it is called a let.

If it does not land in the service box, then it is a fault.