Try signing in from click here instead of using a bookmark or desktop icon.
There are a few other things you can try: Go to browser Tools> Options> Content and be sure the pop-up blocker is unchecked. Set your computer's security setting on Medium. See if the add-blocker can be disabled in your anti virus program, and if using McAfee, it needs updating.
If your computer is set to automatically install updates, a recent Windows update may have changed some of your computer settings.