I was working and suddenly, no internet. I couldn't get it running so I called a tech. He said our ISP, we have only one, had changed their system. We configured the cables differently because what had been my DSL modem was now another router. At least, I think that was the reason.
It worked except I couldnt' connect without an ipconfig /release followed by ipconfig /renew.
I tried another modem and another router. No luck. It worked fine without the router.
I fiddled with it for four days with no luck. My tech came over twice and fiddled with no luck. Last night I really screwed things up so I unplugged both the router and the modem/router. I read for an hour. I plugged them in and reset both to factory defaults. I changed one router setting, gateway to router, and now it works.
I hate it when I don't know why. I also don't know how long it will work but so far it's twelve hours and doing fine.