Why big websites go down: cloud, CDN and DNS failures

Large websites are run by experienced teams on very reliable systems, and they still go down. When they do, the cause is usually one of a small number of things.

Common causes

• A configuration change. The most common cause by far. A setting pushed to many servers at once can break them all.
• A software update or release. A new version with a hidden bug can cause errors that only appear under real traffic.
• Overload. A sudden surge in visitors, such as a major event or a promotion, can exceed what the servers can handle.
• A cloud provider fault. Many sites rely on the same cloud platforms. When one region has trouble, every service that depends on it can be affected together.
• A CDN problem. Content delivery networks sit between visitors and websites. If one fails, thousands of sites can show errors at once.
• DNS failures. If the system that turns names into addresses fails, the site can be perfectly healthy and still unreachable.
• Expired certificates or domains. A simple lapse can make a site unavailable until it is renewed.

Why one fault affects so many sites

Modern sites share infrastructure. A single failing component, such as a login service, a database or a DNS provider, can sit underneath many different websites and apps. That is why unrelated sites sometimes go down in the same hour.

What you can do as a visitor

• Check whether other sites that use the same provider are also affected.
• Read the service’s official status page for updates.
• Avoid repeating actions such as payments or form submissions during an outage.

What you can do as an owner

Monitor from several locations, keep certificates and domains renewing automatically, and avoid depending on a single provider for everything. See uptime percentages explained for what different levels mean in practice.

Related guides

• 500, 502, 503 and 504 errors: what server errors mean
• What is DNS propagation, and why does a change take time?
• Uptime percentages explained: what 99.9% really means

Last updated 2 November 2026. Written by the siteupdown.com team. Spotted a mistake?

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