I use a D-Link firewall. Recently I changed the DMZ IP of the firewall which conflicted with one of the computer's IP. Then I changed the computer's IP. But now I am not able to access the web interface of the Firewall. I tried restarting the firewall. Still no effect. How to make it work again?
Before changing any IP, you can just try to ping it to make sure it is not assigned to another device or PC. This will not work if you have an enabled firewall on the destination device.
If you saved the changes in your firewall config, you should try to access it using the new IP. Otherwise, you should use the old IP.
Also, you may need to wait for some time or flush the ARP table manually.
Try connect using SSH/Telnet and click on system > restore.
This button restore the router/firewall to its factory default settings.