NXDomain is a status of a domain that does not exist in the DNS zone. This means your nameservers are not returning DNS zone information for your domain. There can be many reasons why your domain shows this status, but the main reasons are: your domain is locked by the registrar (clientHold flag in whois), your DNS zone is not activated at your DNS host, your domain is expired or there's a typo in your domain.
The first thing you need to do is contact your DNS host and ask why your domain returns NXDomain.
If everything is fine with your DNS host, check your domain's whois and see if you have "clientHold" status and contact your domain registrar.
If you are using private nameservers hosted by us, you may want to check your domain's DNS zone as well. You can do that by clicking on Actions > Edit DNS Zone. If your DNS zone does not exist or there are errors, click on Actions > Reset DNS Zone. This action will rebuild your DNS zone, and hopefully, it fixes your issue as well.