I once had a similar problem: there were certain sites (DreamHost Panel being one of them) that I could not access. In the end I found that my router had some bad configuration and it was throwing everything out of whack. I had to do a reset on my router and then re-configure all the settings.
But that was not before I spent hours and hours chasing random things that might have been the issue.
Bottom line: most likely a DNS issue. You must figure out what device handles your DNS for the internet
If you have a router handling everything, have you made any changes in your router recently? Flashed it with new firmware? Electric surge might have re-set things?
If your modem is handling everything, does your ISP do auto-updates on that modem?
Rather than making all sorts of random changes (like I did) to your computer network settings trying to fix, think it through a little first, and then try it piece by piece:
If you have a modem, does that modem dish out DHCP to everyone on the network? or does it just act as a dumb gateway and your router handles all network traffic?
Whichever one it is, log in and have a look at the INTERNET settings first. What DNS server(s) is it using? write them down or take a screen-shot, and then try changing them to 18.104.22.168 save the settings…reboot…then reboot your computer as well
Also, clear the cache on your browser…now check…works? okay
Still not working? fine…switch what you just changed back to what it was and then try changing the DNS setting under LAN settings.
it is a pain, but if you trouble-shoot step by step and reboot everything after each change you might save yourself some real head-aches later on.