Tor: Why does one website fail while others work through Tor?

Short answer

The site may block Tor exits, its server may be down, or the current exit relay may have trouble reaching it. If other sites load, try New Circuit for This Site, verify the address, and check the site's status. For an onion service, a typo or offline service is also common. A single failure does not mean the whole Tor network is down.

Sources

Tor Project: Managing identities, Tor Project: Website blocking, Tor Project: Onion service troubleshooting.