Hey there, Jan Bernat
Microsoft has a system that will mark your tenant as inactive if no one signs into it for more than 200 days. Once it is marked as inactive, you will have 20 days to contact support to re-activate your tenant.
If you are within the 20-day window, please contact support using the phone number for your region here: https://support.microsoft.com/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
If it is longer then 20 days, then the tenant and all of its resources are deleted permanently. Once this has happened, Microsoft cannot undo this. This is because it needs to follow Microsoft's data protection policies which can be found here: https://www.microsoft.com/trust-center/privacy/data-management#leave
If this has happened, please can you create a new tenant in Microsoft. You may need to use another Microsoft account to do this, then add your other Microsoft account to that tenant. For help creating a new tenant, please visit this article here: https://free.blessedness.top/en-us/entra/fundamentals/create-new-tenant
Hope this information helps,
Nathan