Sometimes shyt like this just happens. Usually it resolves on its own, but if not try this:
Close your browser and wipe out all of the data (cookies, history, etc.) and then reopen it. 9 times out of 10 it should be back to normal. If not you may have a corrupt version of Java or something. At that point you would need to reinstall any version of Java you have, updating and/or uninstalling old versions in the process.