Try using a 'ping' from the command line to see whether the machine resolves nikongear.net to the correct IP, i.e. 162.210.51.81.
If this is the case, any issue should be with the browser(s). Delete the browser cache, cookies, and any additional security exceptions for NG and the site access ought to return.
However if the machine cannot resolve NG to its correct IP address, the issue might be external, such as you ISP caching DNS records without proper flushing, or an outdated entry in a HOSTS file. The first can be checked by pinging one of our aliases, for example, nikongear.com or nikongear.org since these DNS records likely are older and therefore, already have been flushed. If nikongear.net still is not to be reached, or you end up with Error 404 or similar, the culprit could be a stale HOSTS file. Either edit it, or perhaps even better (unless you have specialised entries therein), just delete the file. Access the NG site in a browser through one of the aliases not used for ping and you should be back into normal operation. (I had this issue on a Win10 box that hadn't been updated for some time).