Another user changed routerlogin pw and user ID. Now what?

[67_gto]

Nighthawk app tells me default user id and pw (admin and password) will not be accepted.  Cannot access router login page.  Appears these have been changed by another user.  How do I defeat this?  thank you 

[microchip8]

Do a factory reset with the Reset Button at the back of the router. Press it for 7-10 seconds then release