Before we start can you tell me if your music was purchased through iTunes or did you import any of your music from other sources such as CD's or downloads?
Also can you tell me what version of OS you are using?
First of all can you try opening your Mac HDD icon from the desktop then going to your User name/Music/iTunes/ folder and seeing if there is anything in the iTunes Music folder so that we can make sure your files are not on your amchine and iTunes is just looking in the wrong place in which case we just need to tell it where to look.
If your files are not there can you open your Mac HDD and tell me if you can see the folder or drive your Time Machine backups are stored on, If you can see it just open the drive or folder then find your iTunes Music folder and drag it to the location I just gave you.
Let me know how you get on.