DIAGNOSIS You're locked out of the web interface. A hardware reset is required.
FIX Find the reset button on the back, near the ports. Use a paperclip. With the router on, press and hold the button for about 8 seconds. Release when all LEDs flash. Wait for it to reboot. Default login is admin/admin.
IF THAT DIDN'T WORK If the reset button is unresponsive, perform a power-cycle reset. Unplug router. Hold reset button down. Plug router back in while still holding button. Keep holding for 10 seconds after power-on, then release.
HEADS UP A factory reset wipes all settings, including your WiFi names and passwords. You will have to set it up from scratch again.