Refer to your routers manual for troubleshooting

for most routers you can open it by resetting your router.

to reset router long press the reset button for 10 second it will restore your router to its factory settings.

refer to your routers manual for its default username & password.

for commom routers. 192.168.0.1 or 192.168.1.1 to open its webpage

username:admin
password:admin