Please make sure that your Java version and internet browser are up to date and that javascript is turned on.
You can also try clearing your browser's cache. The following website has an excellent explanation on how to clear your web browser's cache, cookies and history:
http://kb.iu.edu/data/ahic.html
You might have content blocking enabled in your browser, which could also prevent you from watching the archived videos. In Firefox, if a page contains blocked content, you will see a small shield icon in the top left corner, next to the navigation bar. Clicking this will expand and allow you to refresh and allow all content to load.
In Internet Explorer, a pop-up appears at the bottom of the page automatically prompting you to "continue blocking". Select 'No'.