Are the bitcoins lost for good?
Can you please elaborate on what you mean when you say “goes bust” ? I don’t understand the question.
OK now I understand :)
If you’re Bitcoins are stored on a 3rd party wallet (e.g. Coinbase, Circle or any exchange for that matter) then you don’t really control your Bitcoins, and if that wallet goes bust so do your coins. That’s why it’s advised never to keep large amounts of Bitcoins on those wallets. The best for of storage for large amounts are independent wallets where you hold the keys to the wallet (e.g. Electrum, TREZOR, Greenaddress).
Let’s say one uses Coinbase as one’s wallet. If Coinbase went out of business and could not host their website anymore, what would happen?