The Great East Japan Earthquake hit the Sanriku Coast of Japan on March 11, 2011 with a magnitude of 9.0, causing great damage to the region. However, it was the tsunami that followed which devastated the country, with a final death toll of over 20,000.
Together, the earthquake and tsunami were much larger than Fukushima nuclear plant was built to handle. When the earthquake hit, 3 terminals of the plant shut down because the external power supply was destroyed. For the next hour private generators provided the plant with the electricity it needed, but when the tsunami arrived they were destroyed as well. The last resort was battery power which lasted only 8 hours, at which point concerns rose of a complete core meltdown because if cooling could not be restored the core would melt.
At this time the meltdown was days away and the operators main priority was to cool the core in anyway they could. They began injecting seawater and boric acid (a containment agent) into the core. The process lowered the temperatures to a non-dangerous levels and venting was no longer required.
Sources:
http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/
http://www.world-nuclear.org/info/fukushima_accident_inf129.html
http://www.ilo.org/public/english/region/asro/tokyo/publ/2011disaster.htm
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