dog that caught the car
n. A person who has reached their goal but doesn't know what to do next.
Examples
2003
He entered office in a state of sublime ignorance — "the dog that caught the car knew a lot more about what to do with the car than I knew about what to do with the governor's office" — but he learned fast and had two rather successful terms.
—Jonathon Yardley, “A memoir of Dale Bumpers, a country boy who made good in politics and moved to the big city to stay,” The Washington Post, February 16, 2003
1985 (earliest)
In rare cases, the raider, who has been interested only in making a profit all along, will take over the company. That's usually a case of the dog that caught the car. What does he do now?
—Alan Gersten, “Greenmailer Eying HNG/InterNorth?,” Omaha World-Herald, November 24, 1985
Notes
This idiom is based on the strange habit that some dogs have of chasing cars that are passing by on a nearby road. What on earth would one of these crazed canines do if it actually caught a car? This idiom is also seen as the dog that caught the truck (1993) and the dog that caught the bus (1994).