If you have already looked for your /Music folder, check to make sure it's not just hidden (modify the Windows folder view prefs to view hidden files/folders), but that's a stretch. I don't know the app, I don't know how you used it, but it sounds like something went horribly wrong. I would try contacting the dev to see if they have an answer.
I hope you have your music somewhere else. If not, another lesson learned the hard way.... I feel for you, I've been there. Here is a list of steps to make sure this won't happen to you again:
Steps to digitally keep information (music, pics, docs) that is very important to you.
Step 1: Backup your important data.
Step 2: Backup your important data.
Step 3: Backup your important data.