INternet security issues on some sites

Ever since my mediacom cable internet service has been set up on my pc, some sites will not load, sites like Iowa student loan.org and other sites that require me to log on. I get the page can not be displayed error on IE or timeed out error on Netscape, but most other sites will let me on. I can reach these sites on other computers in the home hooked up to the same cable Internet service. Seems strange I need advice please.