Here's the good news -- a total collapse of the Internet would be almost impossible. The Internet isn't a magic box with an on/off switch. It's not even a physical thing. It's a collection of physical things and it's constantly changing. The Internet isn't the same entity from one moment to the next -- machines are always joining or leaving the Internet.
It's possible for parts of the Internet to go offline. In fact, this happens all the time. Whether it's a particular server that crashes and needs to be rebooted or replaced or a cable under the ocean gets snagged by an anchor, there are events that can disrupt Internet service. But the effects tend to be isolated and temporary.
While there is such a thing as the Internet backbone -- a collection of cables and servers that carry the bulk of data across various networks -- it's not centralized. There's no plug you could pull out from a socket or a cable you could cut that would cripple the Internet. For the Internet to experience a global collapse, either the protocols that allow machines to communicate would have to stop working for some reason or the infrastructure itself would have to suffer massive damage.
Since the protocols aren't likely to stop working spontaneously, we can rule out that eventuality. As for the massive damage scenario -- that could happen. An asteroid or comet could collide with the Earth with enough force to destroy a significant portion of the Internet's infrastructure. Overwhelming gamma radiation or electromagnetic fluctuations coming from the sun might also do the trick. But in those scenarios, the Earth itself would become a lifeless hulk. At that stage it hardly matters whether or not you can log in to MySpace.
The positive way to look at this is to realize that the men and women who helped design the Internet created an amazing tool that's remarkably stable. Even when sections of the Internet have a technical hiccup, the rest carries on with business as usual. While the collapse of the Internet would be a catastrophic event, it's not one you need to worry about.
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