Make sure your device is connected to the router (Wi-Fi or LAN cable).
Use http:// — not https:// — in the browser address bar. The address is 192.168.0.1, not 192.168.1.1.
Try a different browser or clear the browser cache.
If IP Passthrough mode is active (Security > IP Passthrough), the Web UI becomes unreachable — manually assign a static IP to your PC to regain access, then disable IP Passthrough.
If none of the above works, perform a factory reset (see FAQ I forgot my administrator password / How do I factory reset?).