When a favorite app suddenly stops working, it can feel like the digital world has randomly betrayed you. But outages are rarely random. Behind almost every "the site is down" moment is one of a small handful of causes — and once you know them, the whole thing feels a lot less mysterious. Here are the nine most common reasons websites and apps go down, in plain English.

1. Traffic spikes and overload

Servers can only handle so many visitors at once. When a product launch, a viral post, a big sale, or a breaking-news event sends far more people than usual, the servers can run out of capacity. Requests queue up, response times balloon, and eventually the site starts returning errors or timing out. This is why services often stumble at the exact moment everyone wants them — game launches, ticket sales, and tax deadlines are classic examples.

2. A bad deployment

Modern sites ship updates constantly, sometimes many times a day. Most deployments are uneventful, but occasionally new code contains a bug, an incompatible change, or a bad configuration that only shows up in production. The result can be anything from a broken feature to a fully unreachable site. Because deploys are so frequent, they are one of the single biggest sources of self-inflicted downtime — which is also why many outages resolve quickly, as teams simply roll back to the previous version.

3. DNS problems

DNS is the internet’s address book: it translates a human-friendly domain name into the numeric IP address of a server. If a DNS record is misconfigured, expires, or a DNS provider itself has trouble, browsers can no longer find the site — even though the servers are running perfectly. DNS issues are notorious because they can take time to propagate across the world, so the outage (and the fix) can roll out unevenly by region. You can inspect a site’s hosting and DNS details on its status page.

4. Expired SSL certificates

Every secure site uses an SSL/TLS certificate to encrypt traffic and prove its identity — that’s the padlock in your address bar. Certificates expire on a fixed date, and if a team forgets to renew one, browsers will refuse to load the site and show a scary security warning instead. It’s an entirely preventable failure, yet it takes down big-name services more often than you’d expect, precisely because it happens silently until the deadline hits.

5. Server and hardware failures

Physical infrastructure still underpins the cloud. Hard drives fail, servers crash, and memory gets exhausted. Well-run services spread themselves across many machines so one failure doesn’t matter — but a failure in a critical, non-redundant component can still take everything down. Power failures and cooling problems in a data center fall into this category too.

6. Cloud provider and CDN outages

Most of the internet runs on a handful of cloud platforms and content-delivery networks. That concentration is efficient, but it means a problem at one big provider can knock out thousands of unrelated websites at once. When you see dozens of major services all reporting issues at the same moment, a shared provider outage is usually the reason. These events are dramatic precisely because they are so widespread — keep an eye on the live outages page during one and you’ll see the pattern.

7. Database overload or corruption

Behind most dynamic sites is a database doing the heavy lifting. If it gets overwhelmed by too many queries, runs out of connections, or hits a slow, inefficient query during peak load, the whole application slows to a crawl or falls over — even when the web servers themselves are fine. Database trouble often produces the frustrating "everything loads but nothing works" flavor of outage: pages appear, but logins fail and data won’t save.

8. DDoS attacks

A distributed denial-of-service (DDoS) attack floods a site with enormous volumes of fake traffic from many sources at once, trying to exhaust its capacity and push out real users. Large services invest heavily in protection, but a big enough attack can still cause slowdowns or outages. From the outside, a DDoS-driven outage can look just like a traffic spike — the difference is that the traffic is malicious and deliberate.

9. Human error and expired domains

Finally, the humble mistake. Someone fat-fingers a configuration file, deletes the wrong resource, or lets a domain registration lapse. Human error is behind a remarkable share of high-profile outages — not because engineers are careless, but because systems are complex and one wrong command can cascade. The upside is that human-caused outages are often among the fastest to fix once the mistake is spotted.

How to tell which one you’re looking at

You usually can’t know the exact root cause from the outside — only the team running the service can — but the symptoms give strong hints:

  • Many unrelated sites down at once → cloud provider, CDN, or DNS.
  • One site down, error pages showing (500/502/503) → overload, a bad deploy, or database trouble.
  • Security warning about the certificate → expired SSL.
  • Loads on other networks but not yours → not the site at all; it’s local or regional. (See is it down for everyone or just you?)

Why understanding causes helps

Knowing the usual suspects won’t bring a service back any faster, but it changes how you respond. You’ll stop restarting your router during a cloud outage, stop panicking over a certificate warning, and recognize when a "down" site is really just a local hiccup. And when you do want the real-time picture — who’s down, how many people are reporting it, and whether it’s recovering — you can check any service in seconds. Outages are inevitable; being caught off guard by them isn’t.