Most obvious is to check that your ISP hasn't changed its mail settings or set up authentication on the outgoing mail server. Have a look on their support website and check that you have your account set up properly.
Hmmm... Try opening the activity window (I think it's command-zero, or from the menus) and watch what happens in there when you try to send a msg.
Or it could be an issue with the SMTP settings changing, as Applespider suggested. Although usually you should get an error message window for that.
Otherwise it's also possible this is a corrupt preferences file or permissions kind of issue. In which case... you would try repairing permissions (from Disk Utility in the utilities folder) and/or trashing the preferences files (I am not at my iBook, but they should have mail in the name, and they are located in the Library folder that is in your home directory -- what you ought to do is quit Mail, create an archive of all of the files, which have endings like plist and have mail in the name I think, and then delete the originals, and restart mail).