If you mean the 21 December 'end of the world' rumour, this comes from a prediction supposedly in the ancient Mayan (mesoamerican) calendar. There was some uncertainty about the exact date, and also about whether it predicted the end of the world or just a time of complete change, but people picked up on the idea and a little while ago these stories began to circulate.
In the UK and many other places this is mainly seen as a joke, but in some parts of the world it has been taken seriously and people have been stockpiling food, even building shelters and so on. The panic has been particularly bad in parts of the US and of Russia; there have been outbreaks of panic in China too.
From this idea came the rumour that NASA was also predicting a blackout on this date. The idea was posted on Facebook in summer 2012, but nobody knows who started it.