unfortunately searches haven't returned the answer to my question, which is as follows:
foolishly i selected all songs in my library (about 22gb), clicked get info and then checked the blank 'artist' field before clicking ok and closing the lid of my laptop.
i woke up this morning and itunes has done exactly as requested and deleted the 'artist' data from all of the songs leaving

no problem, just undo... undo is greyed out and ctrl+z doesn't work
how can i restore my itunes library to how it was before i deleted the artist data? is the only option to enter the artists manually? that'll take months