What happened when Poseidon caught Odysseus?

Your question asks what happens when Poseidon "catches" Odysseus, so I think you must be wondering about why Poseidon punishes Odysseus and what that punishment entails. I will explain both of these situations, and I hope that this response answers your question.
In The Odyssey, which is the tale of Odysseus's long journey home to Ithaca from the Trojan War, Odysseus and his men encounter Polyphemus, a giant Cyclops who happens to be the son of Poseidon. To escape being eaten by Polyphemus, Odysseus tricks the Cyclops and then blinds him, which makes Polyphemus very upset. Once Odysseus is safely back on his boat, he shouts his name back to the Cyclops, bragging about his victory and unwisely identifying himself. This is a terrible idea because Polyphemus tells his father, Poseidon, the god of the sea, and Poseidon avenges his son, striking back at Odysseus, causing violent storms that make the sea journey home very difficult for Odysseus and his men.