Okay, you will have to do some work by learning how iTunes works so you can troubleshoot.
iTunes uses the iTunes Library.itl to set up a database of track information and file locations. iTunes does not directly inventory the files except when you first add them and if/when you later delete them. The only common way for iTunes to lose track of this database is if you keep your iTunes folder on an external drive and start iTunes while the drive is not active/awake. If this happens just once iTunes will automatically make a blank library in the default location (inside Music > iTunes) on your internal drive. You make no mention either way about an external drive so the only other thing is if your drive is undergoing directory corruption and forgets that the .itl even exists at all. Again your computer does not directly see files, it works through a catalog which tells the computer when you call up file xyz to go and retrieve the data bits stored at location abc on the computer. Frankly I haven't seen anybody complain about true directory corruption in a long time. I suspect there is something about your library configuration which means there is an obvious solution to this, I am just not asking the right, "Oh yes, that's right I do keep things that way," questions yet.
