In order to be entitled to do a chargeback, you have to have a dispute about the transaction. That could be that you did not authorise the transaction (i.e. your card was used without your authorisation) or the goods/services that you paid for were not delivered. You can't do it, e.g,, just because you have changed your mind and no longer want the goods and services, or because you think they are not good value.
When you do a chargeback, the bank debits the merchant's account with the amount involved and credits your account. Basically, they reverse the transaction — that's why it's called a "chargeback". They give notice to the merchant, and the merchant can dispute the chargeback, arguing that the use of the card was authorised, the goods/services were delivered, or whatever. Normally the bank waits for this dispute to be resolved before it credits you, so if the amount concerned has already been re-credited to your account you should, in practice, be safe.
Where the goods haven't been delivered because the merchant has ceased trading or is in liquidation or whatever you can do a chargeback. That could be disputed if the liquidator says that, despite the liquidation, he will honour the transaction, but in practice that's not very likely to happen.
There's a risk that, although the bank debits the merchant's account with the amount of the transaction, they never actually recover it from the merchant because the merchant is insolvent. SFAIK that's the bank's risk, not yours. If the chargeback is undisputed they can't refuse to credit your account because they themselves can't recover the amount involved from the merchant.
