This NASA video describes the near miss of July 2012, when two coronal mass ejections (CMEs) almost knocked Earth "back to the 18th century." It was the most powerful CME Earth has experienced since the 1859 Carrington Event. If it had hit, researchers say we could still be picking up the pieces.
Pete Riley published a paper titled On the Probability of Occurrence of Extreme Space Weather Events, which calculated the probability of such a catastrophe occurring as 12% per decade.
If Earth were to be hit by a very large CME, the grid couldn't be fixed. Not ever.
At this point, a reasonable person might ask how could the power grid go down forever?
The critical hardware that would be damaged in such an event cannot be easily replaced. For example, the 2,100 large transformers of our power grid are handmade and take years to manufacture when our infrastructure is working perfectly. Society would collapse long before all the replacement transformers could be installed.
To learn how society would collapse and to see a video from Ted Koppel and another video from National Geographic about this topic, you can visit my discussion guide.