Nighthawk 6 router stopped working?

What router? You didn't put a model. You put "nighthawk 6" which seems to imply a newer AX device but then have R6400 in the signature line. 

What happened with it? Not working isnt' very descriptive. 

What are the leds on it doing? 

Are you able to connect but get "connected, no internet"

Do wired devices work? 

What modem is it connected to? 

What have you done trying to fix it? 

We need details to help you. We can't help off "stopped working". We're not there to see what happened, what you've tried, or even what your setup is. So help us to help you.