I'm a software engineer...

It is possible the website was shutdown for good, but "the server IP address cannot be found" error means that the forum server is most likely: physically off, is not connected to the internet (e.g., connection was physically severed, internet outage where the server is located, etc.), is undergoing updates, or has crashed (e.g., Blue Screen of Death or DDoS attack).

I can't say how long it will take for the server to go back online because how long it takes depends on who is maintaining the server and I can't predict when they'll tend to their own server, but I would give it at least 48 hours before questioning if it was shutdown for good.

Also, I would recommend periodically clearing your browser cache. It certainly sounds like the site is down right now, but if the server goes back up, you may need to clear your cache so that you can access the site. Just be wary that clearing your cache will clear any shopping carts you have online, etc.. You shouldn't need to delete password cookies, though. The browsing cache is what needs to be refreshed, as sometimes an old cache prevents us from viewing any recent updates/changes to a site (which can give the false impression that a website is still down).