Mine does that from time to time. Usually just refreshing the browser takes care of it. If not, clear files/cookies/etc and try again.
If it still doesn't work, probably is a bug.
If you're using IE8, you need to switch browsers to Firefox, Opera, or Chrome. At the very least, run IE8 in IE7 compatibility mode, which I can't tell you how to do because I haven't used IE in 3 happy years.