iTunes does not look directly at your media files. It uses a database (library.itl) to list what you have added and where the media file was when you did that. If something gets changed such as you moving an iTunes file or folder, or renaming something to do with iTunes, the links get broken. If your drive is corrupt it could also lose files.
If you use iTunes's default preference settings your media should be in Home > Music > iTunes > then the appropriate media folder and subfolder Is your media file there? If it is, locate one of them with iTunes, then iTunes will offer to find the others based upon the same structure. If the media file is gone then something major is going wrong and you will need to restore your iTunes library from your backup you make before updating any software on your computer.