If it worked recently and just stopped working now follow the steps below for clearing your browser's cache. If it never worked for you then most likely you need to update or install Adobe Flash Player.
HOW DO I CLEAR MY BROWSER'S CACHE
1. Close all browser windows and open a new one.
2. Clear your browser's cache including temp files, cookies, and history. If you do not know how then follow the instructions on the help page for the specific browser and browser version you are using: http://www.wikihow.com/Clear-Your-Browser%27s-Cache
3. Reboot your computer and then open a new browser window and go to whatever website you were having a problem with.
4. If you are still having a problem with the website, try an alternative browser such as Firefox. It is always good to have an alternative browser so you can test and check to see if your problem is browser specific. Here is the link to get Firefox: http://www.mozilla.com/en-US/firefox/ie.html
=======================
CHECK ADOBE FLASH PLAYER VERSION: Do you have the latest Adobe Flash Player installed? The latest version is 10.0.32.18. Here is a link that will check your version :
http://kb2.adobe.com/cps/155/tn_15507.html
If you find that you need to update your Adobe Flash Player then please read this article and follow the instructions very carefully. http://news.cnet.com/8301-13554_3-9916676-33.html
If the news.cnet.com link above is not working try then refer to this Yahoo! Help page instead: http://help.yahoo.com/l/us/yahoo/messenger/messenger9/notices/mstaflasherror.html;_ylt=AhwTQlFIGOqJbqSb5vtAsW9f8Sh4
If you do update this also you can uncheck the box for Google toolbar that is on the download page, it is totally optional. After the installation box pops up, close all browser windows before installing. You will need to install Adobe Flash Player in both Firefox and Internet Explorer if you use both for Yahoo Mail. So you copy and paste the link to check adobe flash player into the other browser. Also if you have Yahoo Messenger installed, it needs adobe flash player to be installed in Internet Explorer.