TL;DR - a major solar flare could end civilization as we know it, with recovery measured in decades or centuries.

A [coronal mass ejection](https://en.wikipedia.org/wiki/Coronal_mass_ejection) (CME) or "solar flare" like the [Carrington Event](https://en.wikipedia.org/wiki/Carrington_Event) in 1859, or the comparable [July 2012 solar storm](https://en.wikipedia.org/wiki/July_2012_solar_storm) that hit Earth’s orbit where we’d been nine days before, would be devastating.

From the July 2012 solar storm article linked above:

“A 2013 study[1] estimated the economic cost to the United States would have been between US$600 billion and $2.6 trillion. Ying D. Liu, professor at China's State Key Laboratory of Space Weather, estimated that the recovery time from such a disaster would have been about four to ten years.”

[1] That study was “a joint venture from researchers at Lloyd's of London and Atmospheric and Environmental Research (AER) in the US.”

The effects on our society of a CME and an [electromagnetic pulse](https://en.wikipedia.org/wiki/Electromagnetic_pulse) (EMP) are similar. The US Congressional [Commission to Assess the Threat to the United States from Electromagnetic Plus (EMP) Attack](http://www.empcommission.org/) report from 2004 includes in its Overview:

"Depending on the specific characteristics of the attacks, unprecedented cascading failures of our major infrastructures could result. In that event, a regional or national recovery would be long and difficult and would seriously degrade the safety and overall viability of our Nation. The primary avenues for catastrophic damage to the Nation are through our electric power infrastructure and thence into our telecommunications, energy, and other infrastructures. These, in turn, can seriously impact other important aspects of our Nation’s life, including the financial system; means of getting food, water, and medical care to the citizenry; trade; and production of goods and services. The recovery of any one of the key national infrastructures is dependent on the recovery of others. The longer the outage, the more problematic and uncertain the recovery will be. It is possible for the functional outages to become mutually reinforcing until at some point the degradation of infrastructure could have irreversible effects on the country’s ability to support its population."