The proper way of moving an iTunes library without issue is to point iTunes to a fresh, new folder on your new HDD and then have iTunes move all the associated files....or you get situations like you are experiencing. If you can move your iTunes media back to where it was and then re-point the library to its original location, you will likely have all of your playlists and everything back to normal.
To properly move your library, follow these easy steps:
from
http://blogs.zdnet.com/Apple/?p=4912
"In a nutshell, the proper way to move your iTunes library is to change the iTunes Music folder path in Preferences > Advanced, then select: File > Library > Organize Library (Consolidate Library in iTunes 8).
In iTunes 9 youll see a new dialog box:
Clicking Upgrade to iTunes Media organization organizes your iTunes Music folder into subfolders for Audiobooks, iPod Games, Mobile Apps, Movies, TV Shows, Music, Ringtones, Voice Memos, etc.
Mac OS X Hints notes that once youve checked Upgrade to iTunes Media organization you wont have to do it again (the option will be grayed out) and that it may cause your next music backup (via Time Machine or other backup app) to potentially take a long time, especially if you have a large iTunes collection."