It's hard to accurately say since Japanese officials haven't been forthcoming. The reactors were either already shut down for maintenance or automatically shut down when the earthquake hit. Shut down is a relative term though. After being shut down nuclear fuel remains hot for years. It doesn't release nearly the energy that it does when in an active reactor but it is still hot enough to require it to be cooled for years. The tidal wave wiped out the power plant's ability to cool the still hot nuclear fuel leading it to overheat, cause explosions and release of radioactive smoke and steam. Also of concern is some of the spent nuclear fuel that was in cooling ponds until cool enough to dispose of is no longer being cooled leading to more fires and releases of radioactivity. The reactors are now heavily damaged and crews have had to resort to extraordinary measures to get water into them for cooling.
there are 6 nuclear reactors in jepordy.