Ugh—site outages are the worst, and right now 'fanfiction.net' going offline for some users has a few usual suspects. First, it could be scheduled maintenance or a backend update that the admins are pushing; sometimes updates to the database schema or comment systems force a temporary downtime. Second, a traffic spike — a viral story or spotlight — can overwhelm servers and produce timeouts or 503 errors. Third, there's the unpleasant possibility of a DDoS or other malicious traffic that forces the site behind a mitigation wall.
On the user side, local DNS caching or ISP routing hiccups can make the site unreachable even when the servers are fine. SSL certificate expiration and CDN or Cloudflare misconfigurations are other small-but-gnarly reasons a site can appear down. I usually check 'Twitter' and community forums to see if admins posted a notice, peek at outage trackers, and try a quick DNS flush or a VPN to rule out local routing. If the outage is widespread, sit tight — these things usually get patched within hours, and I end up using the downtime to hunt for older stories I missed. It's annoying, but predictable in one of those internet ways I kind of love to grumble about.