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Typically How the Mighty Have Fallen, especially if they're a victim of their own pride.
If the "whatever reasons" involves an injury, like if they were a pop singer and their vocal cords were damaged in an accident, then Career-Ending Injury.
If they become an overnight superstar and lose that status by tomorrow, then 15 Minutes of Fame.
Edited by KD
Riches to Rags if money troubles might be caused by their drop in popularity.
Princess in Rags if they refuse to acknowledge their change in fortunes, and try (frequently badly) to pretend like they're still rich.
White-Dwarf Starlet if they didn't so much lose it all as just lose the limelight- they might still be reasonably well off, but they've been largely forgotten and long for their days of fame. Usually female, but male examples could be possible- say, a former rap star sitting in a studio surrounded by his gold records and faded promo posters from the early 90s, mouthing along to the recording of the concert where he sold out Madison Square Gardens, trying to come up with that new album that will make the studios beg to have him back.
A character starts off as being famous, popular or incredibly talented at whatever their skill is, them they lose it all for whatever reasons.