Cause: If your website does not work or is down, there are numerous reasons behind it. In this particular case, a running website was down due to expired password of the SQL user. Such errors could not get caught easily, hence we moved our investigation direction to the error logs.
Step1:Logged into web server with administrator credentials.
Step2:At server, Start->Administrative tools->Event viewer, Expanded the Windows logs->Clicked on security
Step3:From the action pane, clicked on find and inserted the domain name, we could see an error log like following:
Login failed for user 'test'. Reason: The password of the account has expired. [CLIENT: XX.XX.XXX.XX]
Step 5: From this error, reason was explicit, hence, we reset the SQL user password and reflect it in the web.config file located wwwroot folder of website.