Then the demand for the crypto currency plummets.
In 2010-2011, I traded BTC for USD in person, with a laptop on wifi. And via PayPal with someone I met on IRC, where there was a bot that kept track of your reputation so the BTC sellers would know if it was safe to sell to you.
At that time the price of 1 BTC was about $1 USD.
I think we'd see something similar, if there was an outright ban.