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Computerworld - Type a URL into your browser, or click on a link. Then you wait for an instant or two, while your page loads. But sometimes you keep waiting. Or you get Error 404: Page Not Found. Possibly, your browser times out while waiting for the server to respond. We tend to summarize all these events with a simple phrase, like "The Internet is down."

In almost all cases, that worldwide interconnection of networks that gives us the World Wide Web, e-mail and countless other online services is still operating. The Internet is still there; we just can't get to it.

But why? The root cause might be almost anywhere. The culprit could be something inside your computer, attached peripheral equipment, a service interruption from your own network or ISP. It could be a regional or wider-scale outage caused by faulty equipment, weather problems, accidental or malicious damage to cables, disruptions caused by malicious software such as viruses and Trojan horses. Or it could be something as simple as the need for routine maintenance.

Possible points of failure

Inside your local system and close by: A hardware component or connection failure is a real possibility. If the hardware is OK, how about software? Windows is notorious for slowing down over time. How long has it been since Windows was installed or since its registry was checked for inconsistencies and cleaned out? Have you installed the latest service packs and patches to your operating system and browser? Is your antivirus, antispyware and firewall software up to date and working properly?

Regional upsets: Maybe the problem lies with your office network or ISP. A quick phone call to the help desk, the network administrator or your ISP will let you know. Their problems could involve hardware or software. Service might be down because of severe weather such as a hurricane, an electrical power outage, or physical damage to a data center or its building caused by anything from an accident to an earthquake to an act of terrorism. It's all unlikely -- and all possible. In July 2007, a major electrical outage in San Francisco took down for several hours a data center hosting a number of popular sites, including Craigslist, TypePad, Technorati and Second Life.

Major service provider outages: Simple things can have unexpected consequences, especially for major service providers. In February 2009, millions of Google users lost access for more than two hours as a result of unplanned downtime caused by testing new software during regular data center maintenance.

Similarly, in February 2008, a two-hour global Hotmail outage occurred, which also affected MSN Messenger and other Microsoft Live services. In July 2001, MSN Messenger was down for up to a week for some users after a disk controller in a database server failed.