Here are a few things to try.
Clear your history, temporary files (this one takes a while – wait for the hourglass to stop) & cookies, restart the computer and retry. . Here is the HELP link for how to do this in different browsers: http://help.yahoo.com/l/us/yahoo/mail/yahoomail/settings/settings-13.html
Also: If it is STILL doing it, scripting may be disabled in your Internet Explorer. This may happen due to updates. To enable scripting, follow the instructions on this HELP page (it is for Messenger, but it may help here as well): http://help.yahoo.com/l/us/yahoo/messenger/messenger75/entertainment/entertainment-35.html
Or try this link to switch back to Classic:
http://us.mg1.mail.yahoo.com/dc/optout?script=no
If none of this works, contact:
Yahoo Customer Service Info:
866-562-7219 or 408-349-3300
http://help.yahoo.com/l/us/yahoo/mail/original/forms_index.html
(live HELP chat link available above)
Yahoo Personal Email: 866-492-4664 (small fee)
8am - 5pm M-F (Pacific Time) [email protected] or [email protected]