On the back or bottom of the router there should be a small hole. You will need to fit something into that hole to press the reset button that is there, such as a paperclip.
With the router plugged in, press and hold the reset button for 30 seconds. After releasing the button, wait for the router to power on, and attempt to login to the router again.
If the above did not work, you can try what is known as a 30-30-30 reset. Get comfortable, because you will need to hold the reset button for 90 seconds. Press and hold the reset button for 30 seconds. While continuing to hold the reset button, unplug the router, wait another 30 seconds, then plug it back in. Continue to hold the reset button for another 30 seconds.