It sounds like what has happened is that you library was read-only, or a permissions restriction was preventing Media Center from writing to the file tags.
When you moved the files outside of Media Center, it saw these as new files rather than using the information contained in the MC database.
Media Center should automatically create backups of your library every few days, so you should be able to use the
File → Library → Restore Library… option to undo your changes.
I would suggest creating a fresh backup of your library before you do this, just in case anything else goes wrong.
File → Library → Back Up Library… (obviously use a unique name and do not overwrite any previous backups)
Now I would normally suggest that you move your files to the new drive using Media Center's
Rename, Move, & Copy Files… tool in the "Rename" mode, but it sounds like you have already moved them to the new drive.
In your case, what I would suggest you do is:
- Go to Tools → Options → Library & Folders → Auto-import and disable auto-import.
- Restore a library backup created prior to copying files over to the new drive. (only restore the library, do not restore settings)
- Select all of the files in your library, right-click them, and select Library Tools → Rename, Move, & Copy Files…
- At the top of this dialog box, select the "Update database to point to new location (no file rename, move, or copy)" option.
- Disable the directories and file name options, and enable "Find & Replace"
- In the "Find & Replace" box, update with the new location of your files. E.g. "Find What: C:\Music\ Replace With: E:\Music\"
Hopefully this will restore all of your lost changes, but it is possible that some of your Album Art may have been lost or overwritten if the only location it was being stored in was Media Center's album art cache rather than being written to the files.