Mail Not Receiving

I Have a Mail server configured with Qmail it was working fine suddenly we are not able to receive mails from other domain. Only internal mails can be sent. I am able to send mails to other domain. Even i am not able to open webmail with the mail.xxx.com/mail if i type the server ip it gets open ex. xxx.xxx.xxx.xxx/mail. I dont have any idea where what has gone wrong as the server was created by someone else. please help me to resolve this issue. Thanks in advance….