But it sounds like a plot line they
should use for a movie - a divine anti-hero who looses his powers through his own fault, then has to struggle as a mortal but still has all the knowledge, insights and contacts of his immortal past, which allows him to see what is
really going on, but has to find 'normal' ways to deal with it all.
The only one in Greek mythology who lost his godhead that I can think of is Chiron the centaur, who was accidentally shot by Herakles with a poisoned arrow, and then gave up his immortality inorder to escape the incurable pain (and is said to have either given his immortality to Herakles, or swapped it for the release of Prometheus).
You could also include Polydeuces, twin brother of Castor, and sons of Leda who were born when she slept with her husband Tyndareus and the god Zeus on the same night. Although they were twins, Castor was mortal (son of Tyndareus) and Polydeuces was immortal (son of Zeus), and when Castor died, Polydeuces gave him half his immortality so that the twins lived one day on Olympus and the next day in Hades.
But both of these willingly relinquished their immortality. I don't know of any god/dess who lost it as a punishment. Maybe the nearest is Poseidon and Apollo being forced to serve King Laomedon of Troy for a year, because they tried to depose Zeus.