"The Billionaire Who Lost His Sun" appears to be the title of a romance novel by Gavin. The story follows characters such as Adriana “Ria” Rossi and Salvatore Moretti, and it is not a reference to an actual historical billionaire or financial event. If you are searching for information about financial loss among billionaires, many real-life billionaires have lost their fortunes due to scandal, poor investments, or legal troubles, but "lost his sun" specifically points to this fictional narrative rather than a real individual.