A hacker has changed my router IP address

Hi David. Your router technically has two IP addresses. An external IP, often only controlled by your ISP, and an internal IP address.

If the external IP address was changed, it was most likely changed by your ISP and no action on your part is required. This is typical and the IP could change at any moment.

If your internal IP was changed (something like 192.168.1.1), my suggestion is to set it back the way it was and strengthen your login/pass credentials to get into your router. If the hacker came in through your WiFi, you should also consider strengthening your WiFi password as well.