Cannot connect to router

Anytime I run into an issue after a firmware upgrade, I usually start with a factory reset. 

Have you tried that? 
What are the led's on the router doing? (color, blinking, solid, etc)

What modem/gateway is it connected to? 

What firmware did you install? 

What router is it. 

There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.