Simple error messages

Hi there
i am new to the IT world, just starting a position as a customer support intern on the helpdesk. I would like to know where I can go to get possible answers to simple error messages, like: The system cannot log you on because the domain (domainname) is not available. In our situation the most likely cause of this is the network cable is unplugged from the back of the computer. Is there anyone out there who can help me with this