How to retrieve GraceNote album artwork?

Hi,

When I was adding album artwork to an album of which was not recognized by GraceNote, I accidentally selected all albums and subsequently added artwork to all 584 albums in my library 😠.

However, when i try to revert them to their former GraceNote covers, iTunes informs me that it is "Proccessing Album Artwork", yet it does not revert/change any of the album covers.

Help please?

Thanks,

mis7ry

iTunes 12.4.1.6, Windows 10