1. Try requesting a new password using the Lost Password feature.
2. Clear your temporary files and cookies. In IE this is under Tools->Internet Options.
3. Ensure that your browser is set to accept cookies.
4. Try requesting a new password (again) with the Lost Password feature.
5. If all else fails, you will almost definitely be able to log in with an alternate browser, such as Firefox (www.mozilla.org/products/firefox).