When talking about Network/OS-only situations, the word "risk" takes on a very particular meaning. Are any possible hazards exist?

Think about the following example: Your company's mail server sends you a message alerting you that your account's password has been changed and requesting your confirmation. You have not changed the password, based on what you are aware of! How did the password change occur, and what may have prompted it? Was there a virus, and if so, which ones, on whose systems, that may have provided the data an attacker required to reset the password successfully?

What does the risk mean in terms of the network/OS system? What are some actual hazards that have occurred?

Network/OS systems have a distinct perspective on what is meant by "risk." Do you have any examples of hazards you could also provide?