What a silly problem. Site is down!!!!

Last weekend I was in my bed and I got a phone call from a client, he was shocked and said to me please help me the live site is down. Opppppps, he got a message that directory listing denied, it seems that he just needs to add a default page in the IIS so I told him to chick this default page he confirmed yes its exists, I tried to follow with him all the suitable scenarios but nothing, finally I decided to go to him to see what is the problem, i logged into the site with the administrator credentials then the site worked? So what is the problem with the subscriber users? Why they can’t access CMS DB? I decided to check all the users on the site manager and on the server. Oppppppppppppps I found in editors group unknown user, who is this unknown user? I asked the administrator, he confirmed that they had a user on the domain but they deleted his account, Oh my god, how come to delete his account while he has privilege as editor on site manager? So this is what makes the site down and once I deleted this unknown user system works fine.Really what a silly problem!!!!