Did you recently make any changes to your browser? Do you get the little pop up window that says to type the first three characters of your city airport country or state? Are you using Internet Explorer or did you recently change? You haven't downloaded the beta for IE8 have you? When I early beta tested IE7 this was very common, I've been using 8 for a week on another machine and have not seen it yet, but it could be.

And of course, if you haven't tried yet, reboot and try again.

Disable popup blockers if you are not getting the popup telling you to enter the first three and try that.