It depends on how they are blocking it. Are you sure they aren't just blocking your login info or was it as a guest? If it actually is the IP address they are blocking, chances are that you have a dynamic IP address (if you don't, there is no easy way to bypass their security) and you could just reset that by resetting your modem. There are also sites on the internet that will mask your IP address though some security measures could easily stop you from doing that. Try a program called Ultra Surf.
To use UltraSurf on Mozilla Firefox, you have to download and install an add-on from the following link: http://www.ultrareach.com/downloads/ultrasurf/wjbutton_en.zip
Extract the "wjbutton_en.zip" file.
Click and drag the "wjbutton_en.xpi" file icon on to the Firefox window.
In the pop-up that opens, click on the "Install" button.
To enable proxy server, double-click on the "u96.exe icon," go to the "Tool" menu in Firefox and then click on "Add-ons."
In the "Add-ons" window that opens, click on "UltraSurf Firfox Tool," and then click on the "Enable" button. Restart Firefox for the changes to take effect.