A lot of people have been trying to answer this exact question. Part of the problem is that we didn't have all the observations that we have today (of the Sun and around the Earth) back then, so we don't know exactly what the conditions were. But the best guess is that it would be pretty bad. It would probably knock out most instruments on satellites, including communication satellites, which means most digital communication would be lost or at least affected. This includes electronic banking, smart phones, internet, etc. as well as GPS. It would change the upper atmosphere of the Earth, causing problems with radio waves making communication very difficult. It could cause significant pulses of current in power lines which could melt the transformers they are connected to, which would result in wide spread electricity blackouts. Fixing all the damage is estimated to take months to years and cost in the billions to trillions of dollars worldwide. However, with enough warning, some of these effects could be minimized. This is why so many people are working to create and improve our ability to predict space weather. It is a tough problem but we're making steady progress!
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