It's possible that there's a content filter on your computer or on some equipment between it and the internet.

If the sites you can't access are all related then it's possible that there's a temporary problem. Also the site can you are trying to access is not loading can be due to the fact that the site is down, on maintanance.

Since you can access some sites and not others, it's less likely to be a problem on your machine but we strongly advice you to scan your computer with antivirus software, maybe this is a virus problem. You can try to use other browser. We suggest using Mozilla Firefox.

Some users solved this problem by modifying the MTU. Check the solution here.

What site are you talking about?