Modem configurations need to be accurate, and if you don't feel comfortable with this, get help. If your modem connection is okay, move to the next level. Sometimes this is difficult to detect. It may even be easiest to troubleshoot with your ISP before you contact the telephone company. If your ISP reports that everything is okay at their end, move to the next level.
Some institutions utilize a firewall to block access to or from some websites.
For example, the government uses firewalls, and if you are working from a US Government computer at an air force base, you may have difficulty that you do not have when you work from home. Chances are the firewall is your problem. You would need to talk to your system administrator if you wish to continue using your office computer. Here the possibilities are almost endless and as the Web grows daily, these possibilities increase. You may have a DNS error.