A technical problem has forced scientists to shut down the huge particle-smashing machine built to simulate the conditions of the "Big Bang" for at least two months.
The European Organisation for Nuclear Research (CERN) said there had been a major helium leak on Friday into the tunnel housing the biggest and most complex machine ever made.
The organisation said it thought the leak was prompted by a faulty electrical connection between two magnets, which probably melted at high current, leading to mechanical failure.
Just 10 days ago, scientists had celebrated the successful start of the Large Hadron Collider under the Swiss-French border, hoping it would revamp modern physics and unlock secrets about the universe and its origins.
In order to fix the problem, the machine will have to be warmed up from its operating temperature of minus 271.3 degrees celsius, spokesman James Gillies said.
"Because the LHC is a superconducting machine that works at very low temperatures, in order to get in and fix it we've got to warm it up, then we go and fix it, and then we cool it down again, and that's a process that's likely to take two months," he said.
Since the machine started up earlier this month, scientists have successfully sent particle beams around the accelerator.
The next step will be to smash the beams into each other to trigger tiny collisions at nearly the speed of light.
This will be an attempt to recreate on a miniature scale the heat and energy of the Big Bang, the explosion generally believed by cosmologists to be at the origin of the expanding universe.