If that's working, check that your network does not block specific sites. Khan Academy videos are hosted via YouTube, so check with your network administrator that youtube.com is unblocked. If your organization blocks YouTube, check out YouTube for Schools, which filters non-educational content. For more information, visit http://www.youtube.com/schools.
If neither of those are issues, check that you are using a web browser supported by Khan Academy: Internet Explorer 9 or above,* Safari, Chrome, or Firefox. Whichever browser you choose, you may want to upgrade to the most recent version.
Finally, YouTube is currently blocked in China and Iran. It's not possible to access Khan Academy from Iran, but there are two ways to access Khan Academy videos from China.
2. NetEase: Khan Academy has also partnered with NetEase for subtitling of videos. Read more here, or check out the content directly at Khan Academy Chinese Zone (http://open.163.com/khan/).
*Microsoft stopped supporting Internet Explorer 8 on April 8, 2014, and Khan Academy will stop supporting it on July 1, 2014.