There may be a couple of answers. The first is to start with your host provider. Software updates can produce unwanted changes to your blog, that can directly effect what the public sees. The second could be straight out, malware. Meaning, someone may have hi-jacked your blog. It happens. My host provider always notifies me when something looks amiss and sure enough, I have found lines of code buried deep in my word press files, re-directing my pages and traffic. In either case, start with your host and back up your files, prior to the day you noticed the problem. Good luck.