I do not know how will it happen as it is equivalent to saying what if networks stopped working and internet gone. Let's us assume your fantasy even it happens then for short period of time u will see some rage moment across the globe and it might lead to big chaos scenario and we might see major accidents.
Cryptocurrency can still vanish without shutting down the internet. It could happen once the government order its citizen to stop using cryptocurrency for certain reasons. Even though most of the cryptocurrencies are decentralized, they can't still stand against the government because it is still mandatory to follow its orders or else you will end up in jail.