Why Can't Login to my Netgear Nighthawk AC1900 Router?

I'm having trouble accessing my Netgear Nighthawk AC1900 router because I'm unable to login to its web interface, and I'm not sure what the issue is or how to resolve it. When I try to access the router's admin page using the default IP address or the router's IP address, I'm being prompted to enter the administrator username and password, but my usual login credentials aren't working, and I'm getting an error message or being denied access, and I'm not sure what to do next.