C:\Windows\System32\drivers\etc\hosts Check (Mac/Linux): /etc/hosts

Malware, a prank, or an overzealous “productivity” tool might have added an entry like:

Settings → Network & Internet → Proxy → Turn off “Use a proxy server.” On Mac: System Settings → Network → Proxies → Disable any active proxy.

Still blocked? Try accessing https://www.facebook.com (explicit HTTPS) or https://mbasic.facebook.com (basic mobile version) – sometimes stripped-down URLs bypass certain filters.