What steps to take if a client does not pay

It might have happened to you, as it does to every salesperson. A customer looks reliable, the deal is closed without any issues, you are already focusing on new challenges, and then the news comes that the client has not paid the invoice or at the very least is late with the payment.

How should you treat this breach of a plan agreed on by the client and the company so as to behave like a true professional?