A mass coronal ejection is more likely than a human-caused EMP but would do the same kind of damage, maybe much more. The thing is, the sun has a much greater capacity to overwhelm our puny human electrical infrastructure and electronics than some measly fission bombs in the atmosphere.
The last great mass coronal ejection, The Carrington Event, was in 1859. Check out the following:
http://news.nationalgeographic.com/n...event-science/