As a powerful storm moved into Virginia on Friday night, lightning took out part of Amazon's cloud-computing service, called Amazon Web Services, which hundreds of companies use for Web-based data storage and computation.

Well-known Internet sites like Netflix, Pinterest and Instagram were not accessible for hours. There was little information for customers about what had happened, or even whether user data was safe.

As the region hit by the storm system grapples with the loss of life and damage to homes and businesses, the interruption has underlined how businesses and consumers are increasingly exposed to unforeseen risks and disruptions as they embrace life in the Internet cloud.

It was also a blow to what is probably the fastest-growing part of the media business, startups on the social Web that attract millions of users seemingly overnight.

Besides Internet-connected computers, tablets and smartphones, which people rely on daily, consumers and businesses are connected to the Web through everything from cars and appliances to utility monitors and surveillance cameras. How well the systems behind these are built, and how they handle unforeseen disruptions, will increasingly affect the larger economy.

Seattle-based Amazon has built a thriving business in cloud computing, with a range of customers including Intercontinental Hotels, Fox Entertainment, Unilever and Spotify, plus 187 government agencies and hundreds of small startups looking for the cheapest possible computing.

None of the big customers reported any service disruptions. While it was not clear if they were using the Amazon server center that was crippled, analysts said the disruption would cause renewed scrutiny of their dependence on cloud computing.

"The way companies view it is in terms of reliability generally," said Michael Chui, a senior fellow at McKinsey & Co. Big customers of Amazon, he said, "have the opportunity to shape the marketplace and make demands that make products better. They will push for improvements."

They will also have another option. On Thursday, Google said it would offer computing over the Internet at half the price of Amazon.

The weekend's disruption happened after a lightning storm caused the power to fail at the Amazon Web Services center in Northern Virginia containing thousands of computer servers. For reasons Amazon was still unsure of Sunday, the data center's backup generator also failed.

Widespread damage

The storm system that knocked out Amazon's center also caused widespread damage over the weekend in West Virginia, D.C., and the capital's Virginia and Maryland suburbs. Virginia officials Sunday increased the death toll in that state to seven. At least 13 people were killed overall — most of them by falling trees — as the storms cut through parts of Virginia, West Virginia, Maryland, Ohio and Kentucky.

Utility crews untangled downed power lines and tree limbs Sunday, working to get the electricity turned back on for about 2 million customers facing a second day of 100-degree temperatures without air conditioning and refrigeration.

Though electricity had been restored to many neighborhoods — more than 3 million customers had initially lost power — authorities said it could be at least a week before everyone had their electricity restored.

About 770,000 customers remained without power in Virginia, among the states worst hit. Emergency workers there and in other states opened air-conditioned shelters in libraries and community centers, and they urged residents to check on neighbors, especially the elderly, who are the most susceptible to the heat.

The National Weather Service described Friday's storm system as a derecho, a fast-moving band that can produce hurricane-force winds.

Amazon said it had restored service Saturday "to most of our impacted customers, and continue to work to restore service to our remaining impacted customers," adding, "we will share more details on this event in the coming days." The company had no further comment.

Previous disruption

It was at least the second major outage for Amazon in that area. In April 2011, a problem in Amazon's networking at a nearby data center took down a number of applications and popular websites, including Reddit and Quora, for more than a day.

To be sure, Amazon Web Services held up better than many utilities affected by the storm.

By Sunday, some Instagram customers in the Washington, D.C., area could, through their cellphones, share pictures on the Web before they had lights or safe drinking water.

Netflix, along with Pinterest and Instagram, did not respond to requests for comment Sunday. Netflix was able to restore service in a couple of hours.

Many more Amazon customers, including the small startups that make up the bulk of its customers, did not report any problems but may have scrambled to make sure their systems were secure.

"We were late at work, playing an online video game that went down, then got a notification from our own system that it was starting to fail," said Benjamin Coe, the founder of Attachments.me, a startup in San Francisco that connects email attachments to online data-storage services.

"We had built for redundancy, but our system failed in two areas, making it hard for the third to work," he said.

Coe said he stayed up until 4 a.m. fixing the problem. It was, he said, the price of working cheaply with a service that enabled his six-person company to potentially serve millions of customers, who he said would be understanding about brief shutdowns.

"For a consumer product that is free, there is an understanding all around that things can fail occasionally," he said. "If you were flying a plane or something, this would be totally unacceptable."

Many startups appear not to take advantage of more expensive redundancy features in Amazon, such as swapping data between the East Coast and West Coast Amazon facilities.

Bigger companies are moving to the cloud as well, but may now look at Amazon Web Services as a stopgap as much as a primary provider.