Which is normally what happens after a civilisation crash.
Getting back to the level of the Middle Ages is fairly easy to do - many, many people can figure out some of the basics even from first principles. Figure out a windmill, weaving, fulling, irrigation etc just from the basics people got going through school probably in just a few years.
Yet the more advanced tech? That's a system fragility. Easy to lose, hard to get back if lost. If there's mass destruction of cities, relocations, perhaps wars and huge numbers of deaths and displaced people why expect a globalised, technological and mainly online world to survive?
Could just as easily find ourselves in the same boat as those in the worlds after the Romans, Greeks, Egyptians etc. Lots of things we can see but have no idea how to recreate. It could take hundreds of years to claw our way back to creating chips. Once the first generation that remembers high school or uni is passed, it gets even harder...
It doesn't look like we had such rapid warming before, at least not in the last million years where we have decent data. We don't know how the ecosystem will handle it.
Also, here's a list at some of the dramatic swings in temperature and sea levels over just the past 2 million years, including many events impacting modern human history, such as the Little ice age: http://www.physicalgeography.net/fundamentals/7x.html