Well, if it's dialing the right number and your login settings are correct then it sounds like a problem with your ISP, or I guess your modem could be screwed, but I need more info.
Have you double checked your DNS numbers and your login/password? Do you get a message like "connection refused" or does it just sit there? Check the TCP/IP control panel and make sure you have it set to connect via modem. That's all I can come up with on the info you provided. Good luck!