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.
If the government really had the capacity to control and manage crypto, crypto were not able to grow until now, you can see a lot of bans have been issued and then repealed, all of which is useless with crypto, that means crypto will not disappear unless the internet is completely destroyed. But the internet is always a very important and secured network, if it is destroyed, not only will crypto disappear, so many other products will disappear similarly, I do not believe that the authorities may be careless in this situation
The decentralized nature of crypto makes it more adoptable by the investors. Governments cannot take control of this market as they cannot keep track of the multiple transactions made by the vast population of investors in this market. Its development with time and addition of investors in the market is the proof of its value and strength. This is also its indication of a strong future and better future of people in crypto.