What causes website downtime? Nine common causes

The nine usual suspects

1. Expired SSL certificate. Renewal failed quietly; browsers now block the site. Catch it: certificate expiry monitoring.
2. Expired domain. The registration lapsed — website and email stop together. Catch it: domain expiry alerts at 30, 14 and 7 days.
3. DNS problems. A changed or deleted record, or a DNS provider outage, stops the name resolving. Catch it: an outside check that reports DNS failures separately.
4. A bad deployment or update. A plugin update or code release causes server errors (HTTP 500). Catch it: frequent uptime checks on key pages.
5. Hosting limits and resource exhaustion. A traffic spike exceeds the plan's CPU, memory or connection limits. Catch it: watch response time trends as well as outages.
6. Expired hosting or billing. A card expired and the account was suspended. Catch it: uptime alerts show the suspended-account page quickly.
7. Third-party dependencies. A booking, payment or font service fails and takes part of the page with it. Catch it: monitor the specific page that uses it.
8. Firewall or security rules. An over-eager rule blocks real visitors or whole regions. Catch it: checks from outside your network.
9. Provider outages. The data centre or CDN has a bad day. Catch it: you can't prevent it — but you can know within minutes and tell customers.

What the list has in common

Almost none of these are visible from inside your own admin panel, and most are preventable with early warning. That's why the most useful monitoring is simple: an external uptime check, certificate and domain countdowns, and an alert you trust.