You may try the domain email address that Microsoft 365 created for you when you signed up with the Business subscription, (for example, user(at)contoso.onmicrosoft.com) using InPrivate/Incognito window with different browsers to directly access the link (https://admin.microsoft.com/) to login Microsoft 365 admin center to check this issue.
If you don't remember that email address/password then I would suggest you contact phone support team they will further assist you from the backend side. While phone call, press "1" as a business email user and press "1" again as technical support. You also need to provide your subscription purchased information such as company name, billing information, phone number etc. See: https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-support?view=o365-worldwide#phone-support