Make sure your browser is up to date and also try clearing your browser cache. If that still doesn't work can you try it with another browsers?
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.