I’m not quite sure the term One Hit perfectly fits what’s being asked for, but I can agree Cuba Gooding Jr falls into that category where he seemed to have tremendous indications he was about to blast off, then it’s like he just stopped on the launch pad and never really went anywhere. I don’t think Cuba had any personal demons that I’m aware of, he just had the misfortune of taking or being cast in a number of not great roles in a row, and his steam went away. Sometimes in Hollywood you have your moment, and you never get a second one. He’s worked consistently ever since but he was like clearly getting hype like a Tom Cruise tier star and he just didn’t go in that direction.
Might Eric Stoltz qualify? Maybe not quite one-hit he had Mask and Some Kind of Wonderful, both considered seminal teenager 80s movies, he had tons of prestige and fame, and he’s largely gone nowhere since Some Kind of Wonderful which was his big pay day following the attention he got for Mask. Note he doesn’t fall into the child actor category–despite playing teenagers he was in his mid-20s during those films, he was 24 in Mask and 26 in Some Kind of Wonderful. He also famously was the original Marty McFly in Back to the Future; they even had done scenes with him before deciding he just wasn’t working out as Marty. Perhaps his career would have had a different trajectory if he hadn’t gotten fired from that movie.