First, please make sure that your browser is up to date by visiting http://whatbrowser.org/
If it is not, please try updating your browser or switching to a different browser.
If it is, please do these four things:
1. Try clearing your browser's cache and cookies: http://en.support.wordpress.com/browser-issues/
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/
Please let us know how each step goes for you, and which browser (and version of) you're using if you're still having trouble.