The particle accelerator, which is buried 100m underground near Geneva, is currently undergoing tests ahead of its proposed restart date later this month, but the testing process was stopped on Monday after the power supply to the collider was cut.
A Cern spokeswoman, Christine Sutton, said scientists had headed above ground to investigate when they made their discovery.
"The problem related to the high voltage supply," Sutton said. "We get mains voltage from the grid, and there was an interruption in the power supply, just like you might have a power cut at home. The person who went to investigate discovered bread and a bird eating the bread."
Sutton said the bird and its bread were discovered at a compensating capacitor one of the points where the mains electricity supply enters the collider from above ground.
The incident cut power to one of the collider's cooling plants, causing temperatures to rise by more than 3C in part of the tunnel.
Superconducting magnets within the LHC require a temperature of 1.9C above absolute zero (-273.15C) to steer, and ultimately collide, particles around the 16.8 mile (27km) circuit.