Others may want to chime in.
The malware that’s on most of your computers can turn your PC into a ‘zombie’ controlled in the background by hackers who aim all of their ‘zombies’ at a given site.
This overwhelms the server/computer hosting the site (too many call coming in to the switchboard) and the site cannot be sent to you for viewing.
In short, “No site for you!”
Denial of Service attack (DoS or DDoS)
A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer resource unavailable to its intended users. Although the means to carry out, motives for, and targets of a DoS attack may vary, it generally consists of the concerted efforts of a person or people to prevent an Internet site or service from functioning efficiently or at all, temporarily or indefinitely. Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers.
One common method of attack involves saturating the target (victim) machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.
I was thinking that someone was tweeting you to death on Twitter... :-)