Imagine a scenario where a sudden and prolonged decrease in hash rate occurs due to a global catastrophe, such as widespread infrastructure and or hardware damage (f.e. because of an atomic war, an unforeseen solar storm, etc). In such cases, it could take many months or even years to rebuild and restore the hardware necessary to reach previous hash rate levels. During this time, the network could essentially become unusable, as no blocks would be found until hash rate levels return to normal.
Let's be realistic: in those scenarios, Bitcoin is the last thing on my mind. Without global network, you'll end up with separate Bitcoin chains: Chinese miners will mine their own blocks, European miners will use other blocks, and American miners will also have their own chain. Once global communications get restored, the longest chain will force a reorganisation on all other continents. Unless there's a manual intervention, for instance because the interests have become too big. In that case Bitcoin may Fork into different versions for each continent.
the network would automatically adjust the difficulty level
If for some reason 99% of all hashrate gets destroyed, a manual intervention can adjust the algorithm with support of the community when needed.
In such extreme scenario, i expect people would try to trade their money for physical goods which makes money no longer valuable.
It's probably going to be gold, cigarettes and booze that's traded for food. Unless the other guy has a gun, then you lose.