What did you do in between the time you added them and the time you found out they were deleted?
Any errors? Blue screens? Did you move folders around? Log in to a different user account?
SOMETHING must have happened that would have caused something like this.
Can you still find your iTunes folder from My Computer?
If you don't know where it is, open iTunes, click Edit, select Preferences, and in the new popup window, click the gear icon that says Advanced. It should show you where the iTunes Media folder is located. Try to navigate to that folder from your My Computer.
Your iTunes folder should be in your My Music folder (possibly called Music on Vista but I've little experience of Vista file strucrture).
In iTunes you need to set it to keep your itunes library organised, then any music you have will be stored in the itunes Media folder in your Music folder. Your music is probably somewhere on your computer, just not organised into the correct place.
In itunes, on the Edit menu choose preferences, on the screen that then appears choose advanced and ensure 'Keep iTunes Media Folder Organised' is selected.
I hope that is all it needs to get you sorted.