Hi I have a shower in a tub and to get the shower on you pull a lever up on the tub faucet. Today the lever didn't work and there was a round thing that fell out of the faucet. How do I fix this? Please help me!!

. today the lever didn't work and there was a round thing that fell out of the faucet. how do i fix this? please help me!!!!
By "faucet" do you mean spout? (see image)

If so simply replace the spout. You can pick one up at any big box store real reasonable. Good luck, Tom