How to you get the Library to remain in the i-Player?

I have been using i-Tunes for over 3 years without any issues until this week. Suddenly, my whole Library disappeared! Fortunately, I have it backed up in two separate locations, however, I have re-loaded it three times now and it keeps disappearing and the only couple of items remaining are a Rush and Night Ranger single that I actually purchased from i-Tunes.

If I go to export Library the files are all there but I cannot get them back on the PC display unless I go through the timely exercise of “Add Folder” (I have over 300 GB of music).

Can somebody please advise me how I can get the library to remain on the interface once it has been loaded?

Thank-you for your anticipated co-operation.