Old ntlworld email account hacked

Hi, I am with virgin media but changed my package and now just have broadband with them. Previously, I was the account holder with my ntlworld email address as the main account holder email and then additional email accounts for my children. I haven’t been able to access my old ntlworld account for ages but my son was still able to. He has seen today that his account has been hacked but can’t access it to change the password (can only get it via his mobile app). I have my old account number but when I try to log in with my ntlworld email address it says not recognised. Is there anyway I can access my old account? And what’s the best way to shut down my son’s old account? Thanks x