I've never heard of this before. The most likely thing is a CSS or Javascript change to the site but if you are certain that nothing whatsoever has changed then you may want to check your server logs to see if your site has somehow been compromised and this is a symptom of it. Alternately you may want to try different browsers to verify the issue in case a browser version is causing it.

If Calendar Nav was previously working and now isn't then something else has changed.