I don't know how iTunes will perform on a thumb drive. It doesn't do so well with a NAS, but maybe USB Flash is fast enough.
The step you took told iTunes to put new music files on the thumb drive.
Next you need to load the original library file, then tell iTunes where you moved all the files.
Hold SHIFT and open iTunes
Click on the Choose Library button
Navigate to your thumb drive iTunes folder, and select the file iTunes Library.itl then click Open.
You should see your library back.
Now play any song.
It will say it can't find it, click the locate button.
Navigate to where that song is on your thumb drive, usually iTunes\iTunes Media\Music\<Artist>\<Album>\<song name>.ext
iTunes may ask if you want to reconnect all your music, click Yes
Wait for 14G of music to be connected.
Once complete, you should be able to play any song, on your thumb drive.
I would set the drive letter so that this thumb drive ALWAYS gets this letter. Otherwise you'll have to reconnect the songs each time the drive letter changes.
Have you backed up your iTunes Library this week? If not, you are risking losing your music and playlists....