The BBC is still investigating precisely what happened, and says it will update its readers once it learns more. For now, it appears the website went offline when some of its routers failed. The routers are responsible for two functions: the aggregation of network traffic from the BBC's hosting centers to the Internet and the announcement of routes onto the Internet that allows BBC Online to be "found." As a result, not only was traffic not redistributed properly to the site, but users' browsers could not even locate BBC Online in the first place.
Major websites typically have systems in place to mitigate situations when their main equipment fails. In this case, however, the backup systems also failed, resulting in all of BBC Online as well as a number of internal services becoming unavailable.
When the site went down, speculation quickly arose that the news service had been subject to a massive distributed denial of service (DDoS) attack. This occurs when hackers crash a website's servers by overloading it with requests, usually from thousands of computers that have been hijacked with malware. The BBC has denied that there is any indication that the failure had been caused by such an attack, at least at this stage of the investigation.
"Thankfully we were able to get most services back up and running within an hour, just before midnight," a BBC spokesperson said in a statement. "The outage clearly didn't go unnoticed, with many of our readers turning to Twitter to talk about possible explanations and using the #bbcblackout hashtag on their posts. We'd like to apologise again to everyone who couldn't get onto the BBC News website last night."