Go to the following URL, to validaate your G Suite administrator domain and user name:

https://accounts.google.com/signin/recovery 

In this way you are confirming you have write access to your domain and so are an administrator for your account by updating domain records. This is the same authentication you performed when you signed up for Google Apps and validated your domain.

Enter an email address to which Google can send instructions to reset your G Suite admin account password.

Click on Continue with domain verification.

Now you must create a CNAME record on your domain registrars DNS control panel. This is on the page where you administer your domain under your domain registrars page. You will need to login at the web page of your domain registrar - GoDaddy, 123-REG, Daily.com etc. and choose DNS configuration. Add a CNAME record with the value Google give you in the above dialogue and Google will scan your domain and confirm the change. 

Click on: 

I've completed the steps above, continue.

It may take up to 48 hours for Google to verify the changes and send the password reset instructions so please be patient.

You will receive an email with the subject line: 

Google Password Assistance

If you follow the link in the message you can select which administrator account to reset the password for. Now you have control over your G Suite business account once more.