"After an interruption of around 24 hours, Block 2 of the Beznau nuclear power plant will start producing electricity again tonight," operator Axpo said in a statement.
The company explained that "a defective safety switch in the non-nuclear part of the plant" had caused the automatic shut-down late Wednesday.
The erroneous triggering of the switch had halted the flow of water into the steam generator, which had caused the reactor to shut down, the company said.
"The fault was rectified by activating an identical reserve switch," Axpo said, stressing that an inspection showed that all the components affected by the defect were now functioning well.
"The Swiss Federal Nuclear Safety Inspectorate (ENSI) approved the restart of Block 2," the company said.
Switzerland reacted swiftly to the nuclear disaster in Fukushima last year, with parliament deciding to phase out nuclear energy.
Under current plans the country's five reactors will be put out of action by 2034.