This problem may be caused by any the following: (you may stop troubleshooting when the problem is solved)
1) Reboot your computer
2) Active scripting and scripting of Java Applets not enabled
In IE click Tools > Internet Options
Click Security Tab, click Internet to select Internet Zone
Click Custom Level
Scroll down to section labeled Scripting
Select Enable for Active Scripting
Scroll down select enable for Scripting of Java Applets
Click OK (Are you sure? Yes)
Click OK on bottom of Internet Options window
Close and relaunch your browser
Try Yahoo! Mail again
3) Update Flash. Yahoo! Mail uses Flash technology. Updating to the current version of Flash can resolve many issues that can occur in Yahoo! Mail:
Install the current version of flash player:Install Adobe Flash Player
4) Clearing your Internet browser cookies and cache
Click Tools > Internet Options
Click on Delete under Browsing History
Place a check in the Temporary Internet Files and Cookies checkboxes
Click the Delete button
Click the OK button at the bottom of Internet Options
Close and restart your browser.
Try Yahoo! Mail again.
That is as far as I will go for now. There are more things to try, but I would like you to try these solutions first. Let me know if you are unsuccessful.