Bitcoin is a digital currency that circulates freely, without any control or supervision from banks, governments, or anyone else. In contrast, bitcoin relies solely on cryptography and software between computers.
what if the government closes or does not allow the exchange of Bitcoin or cryptocurrencies with the official currency in each country?

Still you can sell your bitcoins to someone for fiat in personal or via peer to peer exchanges,.there is no practical way to stop the trading of bitcoins.