Did they start sending NXFAIL for DNS requests for that domain? That's what "turning off the domain" means. But in that case, API access via IP would continue to work without issues.

Or did they start blocking traffic to the IP that you had associated with the domain? In that case, the domain would continue to work, you just need to switch the IP.

Based on your information so far, I wouldn't know what to do to repair it. I'm not surprised that others were unable to help, too.

Generally, I would recommend not hosting APIs on the primary domain for exactly that reason - it's too easy to be hit with some sort of complaint and have that domain cut off (DMCA, preliminary injunction, SPAM complaint against your mail server, shitty host, ...).