Technically you could've still sold it even when the chain was "halted", since there were still a few exchanges that kept their various Luna markets open, and centralized exchange trades happen off chain. The problem was that you couldnt transfer your Luna from your private wallet to exchange wallet.

Now the chain has resumed, and you can transfer Luna between wallets, but they have disabled on chain swapping between Luna and UST, and closed the IBC channels which allow you to "bridge" your Luna or UST from the Terra chain, to another sovereign blockchain.