Spain nuclear body said the country's oldest nuclear plant suffered an unexpected shutdown because of a problem with the main generator.
The Nuclear Safety Council said there was no danger of a radiation leak or environmental damage from Friday's stoppage at the northern Santa Maria de Garona plant.
It said security systems were activated correctly and the problem was being investigated.
Greenpeace has long demanded the plant be shut down for safety reasons.
Garona opened in 1970 and is nearing the end of its 40-year working life. A decision on whether to keep it open for another 10 years is expected in June.
Spain has six nuclear power plants supplying up to 30 percent of Spain's electrical energy. A moratorium on new plants was imposed in 1983.