If the hardcoded domain name changes, then the app needs to be recompiled with new domain name and that's it. The service is instantly restored and no data needs to be updated to reflect the change. In fact, the server side doesn't even need to be made aware there was any change. It just continues serving data as normal.

If you have a blockchain, and the blockchain requires a key to write to, and there's only one key, and the private key gets stolen by malicious email attachment and posted on darkweb - what now? Do you need a whole new blockchain? What happens to data from the old blockchain, how do you verify the new data is correct? Or do you keep the blockchain and use the leaked key to revoke the key and set a new one? How do we know the new key is legitimate?

I genuinely don't know how to recover in this situation and would really like for someone to walk me through the process.