If you are not running one of the browsers and versions listed at http://browsehappy.com/ please try upgrading or switching first.
Note: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html
If you are running one of the browsers and versions listed at http://browsehappy.com please do these four things:
1. Try clearing your browser’s cache:
2. Make sure that you have third-party cookies enabled: http://en.support.wordpress.com/third-party-cookies/
3. Try with all browser extensions or add-ons temporarily disabled.
4. Try enabling HTTPS: http://en.support.wordpress.com/https/
If it’s still happening, let us know which browser (and version of it) you’re using by checking here if necessary http://supportdetails.com and also post any error reports you get.
Once we know you are up and running with a current browser version, please read this thread http://en.forums.wordpress.com/topic/changes-to-media-handling?replies=1 and then post any specific issues you have here and we will help you with them.