Happened to me just yesterday, trying to get back into an old web serial I used to follow. The 'story unavailable' message popped up, and I immediately assumed the site had just taken it down. But after some digging, I realized it was a regional block—the author licensed it for a specific country's platform, and my VPN had turned off. So the content was there, just not for my IP address at that moment.
Sometimes it's way more straightforward, like copyright strikes. I've seen fantastic fan translations vanish overnight because the original publisher finally noticed and issued a takedown. The story isn't gone from the internet, but that particular link is dead. Other times, the author themselves pulls it for revisions or before a professional publication. It's frustrating, but it happens.