The Blackwall

June 9th, 2022, a timer ran out after the death of Rache Bartmoss and his magnus opus was released on the world: the DataKrash virus. It exploited not just the code underpinning every part of the NET, but how that code could execute, and it would grow into pure chaos. Strike one.
The DataKrash virus wasn't the only nasty surprise from Bartmoss, however, and it's unlikely even he could have foreseen the devastation he'd wreck. R.A.B.I.D.S spread like wildfire as they made copies of themselves and attacked anyone near their systems. They developed and became more and more vicious, but also better at infiltrating new networks. Anyone who connected to a server put themselves and all those who they might communicate with at risk. Strike two.
NetWatch first thought the corruption came from individual hackers around the world. They investigated the possibilities for system errors. By the time they realized the scope of what they were dealing with, it was too late. They tried to fight it, but eventually, all they could do was to attempt to shut down the NET. Connecting was banned in 2022 and NetWatch declared it officially down in 2035.
Officially down doesn't mean actually down. Alt Cunningham was still running Ghost World for engrams and AI, for example. Some such entities even worked with NetWatch to try to stop the more destructive ones. That fight became ever more intense as the worst AIs were improving and killing humans frequently. Those programs were optimizing for destruction, and NetWatch decided that they had to as well. Strike three, they concluded.
The Blackwall is an insanely complex integration of communications protocols, an AI-like core, and hardware adaptation. It exists in all networking equipment since the late 2040s and inserts itself if it's not there already in something that connects to a network. It's job is to instantly create a split between things that carry any trace of the old NET and things that don't. And then it will never allow those sides to interact. Code that tries to penetrate it is deleted along with anything else that might be suspect. The connected mind of a netrunner is cooked to charcoal.

The Benefits

There is once again a global NET. From CitiNets grew interconnectivity an once again, one can do international trade or pirate music from obscure European DataPool. There are hundreds of little things during a day that are made that much easier thanks to the new NET, from talking to friends to learning about customers through how they talk to their friends.
The new NET has some limitations, yes, but it's better than nothing.

The Dangers

Cyberspace beyond the Blackwall isn't gone. It's just hidden. Behind that curtain are extremely advanced programs that are putting all their effort into bypassing it. Of course, the resources behind the Blackwall are limited. No one, or at least no humans, are building new servers for them or repairing old equipment. They only have so much drive space and energy to consume. It's about survival of the fittest, and it's unlikely the winners in that fight will be ones who would treat humanity very well if they ever push through.
When they push through.