The ride was stopped by staff at about 12.00 BST, with about 30 people stuck on the ride.
A spokesperson for the theme park in Staffordshire said: “There was an evacuation and all guests are now clear.”
In a statement, the theme park said: “Alton Towers Resort stopped The Smiler to investigate claims a piece of debris had fallen from a carriage.
“At no time were guests on the ride at any risk and all were safely removed from the ride promptly by staff, in line with our comprehensive standard procedures.
“As the health and safety of our guests is our priority, the ride will remain closed whilst the Resort’s techincal team investigate the matter.”
In June 2015, one carriage of the Smiler rollercoaster crashed into an empty static carriage, leaving five people with catastrophic injuries.
Two women, Leah Washington and Vicky Balch, had to have their legs amputated.
The company pleaded to breach health and safety regulation and a two-day hearing is scheduled for 26 September.
