Nothing hapens immediately.. The threat of NN causing the internet to not be available at all is silly. NN is about the Internet Service Providers possibly throttling speeds from content providers if there is no traffic deal between them.
Today, the content providers have petabytes of data like streaming video. In order for you to watch that, you need a service provider to connect your viewing device to the content. The ISP does that. Practical application: Netflix has a movie you want to watch at home on your tablet. The tablet connects to wifi which connects to the router which connects to the ISP - in this example, let use Comcast. Comcast has no formal/legal agreement with Netfix and downrates Netflix traffic to be slower than Hulu... and Comcast has a formal agreement with Hulu. The Netflix video buffers more than the Hulu video because it has slower throughput. You decide you don't like Netflix and go elsewhere... like Hulu or others. No Netflix' fault - though they could have worked out some sort of agreement with Comcast and the traffic would not be down-rated.
In this first case, with NN in place, ALL internet traffic is the same so all the packets are treated equally.
The interesting part is that anyone who pays for internet service has already instituted their own "downrating". I used to have a dSL line that provide 1.5 mbps. Video had lots of buffering and file uploads and downloads were slow. I chose to upgrade to an 18mbps downstream. Now, video streaming is not an issue. I get what I pay for.
Proponents of NN say the internet should be free and all data packets treated equally.
Opponents of NN say nothing is free and you should get what you pay for.
Why can't I have a 100 mbps internet connection to my home for free - or for the price of a dSL line? The router, cables, switches, office space where the network gear is located, the power back-up systems, the HVAC systems, the security systems, the connections to the internet, the people who design, install, maintain the hardware, firmware and software for all this + all the other finance, human resources, administration, labs, worker office space, employee benefits and more all cost $. And that is just the ISP side. The same goes for the content provider. In this case, the content provider wants full use of the ISP's infrastructure.
That's like NFL football (or whatever other content provider you want to insert here) wanting to use Dish Network's delivery mechanism either with no agreement or at the same rate as when standard definition video was carried - but transmitting 4k video (much larger bandwidth, much different/new equipment requirements, etc.)... and the homeowner expecting a free upgrade to the dish on their home and receiver connected to their TV...