This was caused by an automated cleanup process we occasionally run. The way that's supposed to work is that a domain that does not resolve or resolves to a non-HelioHost provider, and is NOT using Cloudflare, was supposed to be disabled to improve server performance.
The cleanup script had a few false detections, you're the second person to report a domain inadvertently disabled. When we designed it we expected Plesk to let you turn it back on yourself, but it doesn't (I wonder if we could alter the script or Plesk's database somehow to allow users to restore their own domains?)
Either way, this domain has been restored and may take up to 2 hours to work.