I cleared the DNS cache last night right before going to bed (and then subsequently ended up losing connectivity from the modem). Rebooted the modem this morning and regained connectivity, but I still couldn't access Checkmated.
Right now I'm using the wifi at work to connect with my netbook and I can access CM with no problem. It's so weird.
Try going to Google (or other search engine) and searching for checkmated.com or forums.checkmated.com. Click on the link and see if it will load for you. If not, clear your DNS cache and try the Google link again.
I haven't been able to access either site for months - I had assumed they were defunct. I've tried periodically, and again whenever I've seen a reference to getting the site(s) up and running on LJ, from either a PC or a Windows laptop, always with the same result - DNS fail. I've tried again today, using IE, Firefox and Chrome, and even tried it using Midori running under Linux on a Raspberry Pi - no go.
It looks as though the URL is not getting propagated. I don't think this could be down to local individual DNS caches. Over the period in which I've not been able to access the sites my router has been rebooted more than once, my PC and laptop are switched off whenever they're not in use and, as it happens, Linux on the Raspberry Pi was re-installed from scratch yesterday.