The prediction in the PEW report that the most respondents agreed with was something I'd never even thought about before -- the possibility of a "devastating attack" on the infrastructure of the Internet itself.
I would not have imagined that such a thing would be possible, since the physical side of the Internet is servers, computers, network connections, and power supplies all over the world. Could a few targeted attacks to major servers cripple the whole system? Or could a clever virus spread across the Internet quickly enough to shut everything down? If these things are possible then I'm sure someone will try, but I would have guessed that the Internet is decentralized enough to be safe. I'll admit I don't know a lot about these things, though.
How do you think people would respond to a massive Internet blackout? When I was an undergrad we occasionally suffered a campus-wide loss of Internet service, due to physical problems with the cables connecting us to the outide world (we were located in a small town in the mountains). This was frustrating and annoying, but mostly because it deprived us of a valued source of entertainment. However, we knew that the problem was local and they'd get everything working again within a couple of days.
In the wider world I don't think there are yet many crucial services that depend on the Internet, so life could continue mostly as usual if there were an Internet blackout. Some people wouldn't have anything to do at work, but no one would die from it. My chief concern would be that people would panic if they knew something had gone wrong but couldn't rely on their usual source of instant, searchable news to find out what had happened. But if TV and radio were unaffected, the Internet blackout would surely be the big story on all channels.