MS have locked me out of my own, private domain and nobody seems to be able to help

As your issue description is primarily written in English, I will provide my response in English for your convenience. Please feel free to let me know if you would prefer the information in another language.

Hi Are,

A custom domain can only be added and verified in one Microsoft 365 / Microsoft Entra tenant at a time. If your domain was previously added to a tenant, Microsoft will treat accounts using that domain as organizational accounts and may direct you to contact the administrator of that tenant.

If your domain has already been registered in a tenant that you don't have access to, you'll first need to contact Microsoft Support and prove that you are the rightful owner of the domain before they can help recover or release it.

You may be asked to provide information such as:

• Proof of ownership of the domain
• Access to the domain registrar account
• Access to DNS management for the domain
• The ability to add or modify DNS records requested by Microsoft
• Domain registration details and billing information
• Any historical Microsoft 365 subscription, tenant, or administrator account information that may still be available

To contact Microsoft Support, you can try the following:

1/ If you currently have access to another Microsoft 365 tenant with an admin account

• Sign in to the Microsoft 365 Admin Center using that administrator account, then go to:
• Help & support (bottom-right corner of the page). For reference: Get support for Microsoft 365 for business

From there, you can create a support request and explain that you are the owner of the domain and need assistance recovering or releasing a custom domain that appears to be associated with another Microsoft 365 / Microsoft Entra tenant.

2/ If you cannot access the Microsoft 365 Admin Center

You will likely need to contact Microsoft Support by phone. You can find the number for your region here: Contact Microsoft customer support. (work account tab)

Tips for navigating the IVR: 

In some countries, an automated voice system is used. Example of a dialogue:

• IVR: What problem is bothering you?
• Client: Lost access to the only admin account
• IVR: What product do you use?
• Customer: Office 365 for business
• IVR: School or Business Account?
• Client: Companies
• IVR: Are you an administrator?
• Client: Yes
• IVR: Is there another administrator in your organization?
• Client: No. I am the only admin in my tenant    
• IVR: Do you need to create a support ticket?
• Client: Yes. I need to create a ticket. Please send me directly to the Data Protection Team.

3/ If you are unable to call Microsoft Support

Consider signing up for a trial subscription to create a new tenant (Select "Try for free" under a business subscription). Once set up, you can access the new tenant's admin console and submit a support ticket to speak with the data protection team on behalf of the previous tenant. Once your issue is resolved, please remember to cancel the trial subscription to avoid any unintended charges.

4/ If your organization's subscription is from a partner or reseller: Contact the reseller's support provider to help open a service request on behalf of you instead.

I hope this helps point you in the right direction.