The fateful day would be at the very least decades in to the future, likely closer to a century, when the Internet became serious. As in, not "SERIOUS BUSINESS" - serious, but "I'm going to stab you and take your wallet" - serious.
See, in those days, two things happened almost simultaneously: First of all, every single piece of knowledge was now available from the Internet. All games, movies, books, just about everything was bought online, legally, and stored on your hard drive - which was also online, incidentally, so technically it was more like a "soft drive" or something now. Computers were online by default, 24/7, even while they were shut down. And sure, people still had books and DVDs up hidden somewhere in the attics, but no one used them anymore.
Secondly, humans themselves were always connected to the computers themselves, and through that, to the Internet: The computers they used had become nothing but an extension of their own brain, like a car used to drive across the highway of knowledge. An epic level hacker could, theoretically, hack himself into other human being's mind, but people also had pretty kickass firewalls, so none of that has ever happened - not documented cases, anyway, there were always rumors and strange incidents.
And then the Internet became self-aware. Hell, it was just a matter of time.
All of the sudden, humans noticed that they weren't in our world anymore: They had been physically transported into the Internets. Or rather, their minds had been sucked out, "uploaded", while bodies were now just empty shells left behind. All the while, swarms of terrible monsters, hereby known as Cybers, that had spawned in the depths of the web eons ago, suddenly appeared and started to hunt down poor people. Many were killed, either by being caught by surprise, or simply for being weak and unworthy for living in this new world.
Those who survived the initial sudden attacks soon formed great societies to the most popular and respected old websites: Places like Wikipedia, Something Awful, and 4chan, became final safe havens from Cybers, the great cities known as Webforts. Meanwhile, most other smaller websites, dead or unpopular, small forums, and morally questionable sites such as porn and pirating (popular or not), were quickly overcome by feral beasts, and everyone inside was either eaten or forced to flee for their lives. Some did survive, albeit usually very barely.
Incidentally, when talking about "morally questionable", some of 4chan's boards didn't survive. All porn and especially /b/ were overcome and destroyed. /r9k/ survived with heavy casualties. The parts that remained, however, thrived, although the particular site doesn't have very good relations to the more "civilized" places.
The Internet itself, after becoming sentient, very soon split into smaller entities. They became the new gods of this realm: Some of them benevolent, others evil assholes, some neutral, etc. They were worshiped by humans, or even the Cybers, granting them powers, protection, and whatever gods usually do. Each webfort, apart from smaller cults worshiping various other gods, had a patron deity of their own: 4chan, for instance, worshiped moot, who had already uploaded his entire mind to the web decades ago, following the death of his physical shell, and was now a literal god, dedicated in upholding peace and justice within his realm.
Traveling between websites was dangerous at the very least: Most of the pages were dead or nearly uninhabited, and Cybers hunted humans down everywhere. Travel was done mainly through links. Unfortunately, this was far more difficult than it may sound, because while internal links at the same server - such as moving from /tg/ to /v/ - worked perfectly, everything else was seriously messed up: External links had warped and twisted into randomness, no one knew where they led to, and Cybers liked to pour inside from them occasionally. Smaller sites had to withstand raids almost daily, not only by Cybers, but by opportunistic and evil humans as well.
Yet exploring would be crucial for human survival. The mankind was separated and lost, being reduced into small or slightly larger villages - and an occasional webfort, as mentioned - isolated from the outside web, waiting to find each other. Also, 99% of all knowledge was still hidden outside, waiting for its discovery in the depths of the dangerous web. Meanwhile, webforts themselves were full to the brim, and so many websites were so full of precious and important resources, requiring humans to settle nearby and take advantage of them. Therefore, people formed adventuring parties, caravans, warbands and such, and boldly wandered outside, in search for fame and glory!