If there would be no liquidity for the BTC, maybe gold would be a better option? You can always sell gold...
BTC relys on the internet, the bank debit card was just a optional service giving you a branch of ATM. So if the bank was stopped you'd not get the notes but it doesnt mean the btc is lost until the bank is forced to forfeit to government.
BTC could still be used with no cash, it would just become pure trade exchange like the first pizza for btc. There would be no reliable price in dollars or euros. Its possible this scenario sees the highest ever worth for bitcoin as people cannot transact any other way reliably and BTC is not part of reserve banking, fiat, debt or bond markets or subject to IMF jurisdiction.
That all sounds like a positive, cold water in an earthquake suddenly becomes a unique resource right. That brings us back to the internet, can it suvive disruption. If the net stayed up then btc could continue, some countries are this bad already. Maybe ask someone in Venezuela