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First, I second the nod for Malwarebytes Anti-Malware heartily. Second, being familiar with Sibelius, I find it highly unlikely that is the source of your problem. Nonetheless, it is important to realize that often times, the site you happen to have been on when you experience a sudden change or symptom in your system, may or may not be responsible. Many malicious programs reside in your temporary files for a bit before finally being executed at a later point. Sometimes, hours, other times days or even months.
Finally, -and the most significant reason for my additional post on this point- currently there are several flavors of malicious code at large that will first, set the attributes of your personal files to be hidden. They will still be on the system, just not visible via the default settings of Windows Explorer. If left to continue its aggravation, the code will do the same for all of the files on your desktop and on to the typical default files necessary for all users which normally reside in the default user profile. Left long enough, it will even begin deleting those files. It can be a real pain to clean up, and though MBAM (Malwarebytes) will find and clean the offending code from your PC, it will not fix the issues caused by this particular coding.
So before you get distraught over the loss, be sure to enable the ability to view hidden files within windows explorer. Open Windows Explorer go to Tools/Folder Options in the file menus at top. Click the View tab on the property sheet, put a tic in the settings to "Show Hidden Files and Folder" and untic the option to "Hide All Protected Operating System Files". Then check to see if your folders and files are now visible. If so great. If not, depending on the relative importance of recovering the files in question, you might consider shutting the machine down and asking for further assistance from a different machine. Do run a scan with MBAM first though. Also, MBAM may be installed, updated and run from Safe Mode With Networking. Good luck.