The governments can block any currency that is centralised . For non centralised coins they can disrupt those that use exchanges but if as in the case of BTC they close exchanges we just use Peer to Peer . Interesting example of attempts to ban Bitcoin is Nigeria where the end result has been an increase in the publics interest in it and an increase in the price
This exact scenario was anticipated by Nakomoto and the cypherpunks and appropiate precautions taken. You might think if they hold you prisoner you would give over the keys but
The most likely scenario is the use of or threat of force to gain keys . To overcome that always have more than one wallet and give them a substantial but smaller amount than what you have. The chinese government tried to stop Bitcoin back I think in 2017 but they gave up in the end . If they cant pull it off what hope has a liberal democracy
