If a day without Wikipedia was a bother, think bigger. In this plugged-in world, we would barely be able to cope if the entire Internet went down in a city, state or country for a day or a week.

Sure, we'd survive. People have done it. Countries have, as Egypt did last year during the anti-government protests. And most of civilization went along until the 1990s without the Internet. But now we're so intertwined socially, financially and industrially that suddenly going back to the 1980s would hit the world as hard as a natural disaster, experts say.

No email, Twitter or Facebook. No buying online. No stock trades. No just-in-time industrial shipping. No real-time tracking of diseases. It's gotten so that not just the entire Internet but individual Web sites such as Google are considered critical infrastructure, experts said.

"Nobody would die, but there would be a major hassle," said computer security expert Mikko Hypponen, chief research officer at F-Secure in Helsinki, Finland.

If an Internet outage lasted more than a day or two, the financial hit would be huge, with mass unemployment, said Ken Mayland, a former chief bank economist and president of ClearView Economics. Eugene Spafford, director of Purdue University's Center for Education and Research in Information Assurance and Security, worries about bank runs and general panic.

Psychologically, too, it could be wrenching.

"I think it's easier to get off heroin," said Lisa Welter of New York City, who weaned herself for a month last year from just the social aspects of the Internet -- she still paid bills online -- and felt as if she was "living in a cave."

"There would be a sense of loss: What would I do with my time?" said Kimberly Young, a psychologist who directs the Center for Internet Addiction and Recovery.

On Wednesday, certain Web sites, most prominently...