It's probable that you try to open the page from a link (favorite, bookmark,...) that got changed to some unvalid location.
Check that the link that you are trying to use is correctly formated. If needed, enter it manualy or just open the general Yahoo! page and, from there, go to your mail acount.
Open: www.yahoo.com
Log in and access your e-mail.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix email.