Extreme ultraviolet light streams out of an X-class solar flare as seen in this image captured on March 29, 2014, by NASA's Solar Dynamics Observatory. This image blends two wavelengths of light: 304 and 171 Angstroms, which help scientists observe the lower levels of the sun's atmosphere.(NASA)
Well, if you want to be really picky about it, a solar flare can't hit the Earth. It's an eruption on the surface of the sun. But a big flare can send a solar super storm hurtling towards Earth, blasting us back into the days before electricity.
It's hard to describe how much scary stuff comes out of a solar super storm – hot gasses moving at near light speeds, deadly radiation, insane magnetic fields, you name it. We had a taste of a pretty big solar storm back in 1859 when it knocked out the entire telegraph system in the U.S. and literally jolted some of the operators out of their seats.
Fast forward to today, where we're so completely dependent on technology that things can get really ugly. A storm as powerful as the 1859 version will probably fry a lot of satellites and our electrical grid. So millions of people could be without light, water, fuel, sewage treatment, perishable food and meds for months.
And not to mention money. Your cash machine will be kaput.