Pretty much it's the Four Horsemen of the Apocalypse: War, Famine, Pestilence and Death.
Any sort of collapse is going to lead to a lot of people fighting over dwindling resources. It's also going to provide fertile ground for warlords to take over and run their own kingdoms. And those kingdoms can be expected to fight. The collapse of the Western Roman Empire is likely instructive here. You will have a hollowing out of the cities as the logistical chains supporting them collapse. Where they make sense, the ruins will be used by smaller populations and in surprising ways (stables, forts, ramshackle dwellings). Subsistence farming will again be the occupation of the vast majority of the population. Though, that's going to take time to get going again; so, expect a lot of people to die of starvation. And, of course, farmers will regularly find themselves subject to raids from the previously mentioned warlords. Some might be lucky enough to be left with enough food to keep feeding themselves and just become serfs to said warlords.

With modern medicine and vaccines gone, our old friends tuberculosis and small pox likely show back up and start taking their toll. A lack of sanitation and water treatment brings back cholera and dysentery. And then there's all the joys of bacterial infections, without anti-bacterial medications. So, one unlucky scratch and you get to die a horrible death or face an amputation without the benefit of anesthetic.

So ya, pretty much we get to go back to scrabbling in the dirt for the hope to not die horribly. And maybe, if we're lucky, society will put itself back together again, eventually.