my dns went bye-bye

I used my wired internet one day, come back and try the next day and I can't logon to any sites, even my routers config/home page. Got fed up with it. Come back a week later and try again, still no internet but this time I can see the router page. This time I notice there is no dns server listed in my network connection settings. Adding 8.8.8.8 and everything come back of course.
Any ideas why it just upped and forgot my existing dns (whatever that was set to, because I don't know)?