the wheels came off the bus

Some endeavor or activity failed or went terribly wrong, especially after some unexpected problem or mishap arose. The team was looking primed for a championship run at the start of the season, but after their goal keeper went out with an injury, the wheels came off the bus. The show was one of the hottest programs on television when it debuted. But then the wheels came off the bus in the final season, and now it's often derided as one of the most disappointing finales of all time.