Bin Apple and bin ITunes,
Not the most helpful comment given that his music collection was in iTunes! If you lose a hard drive then, without backups, you will lose your music collection whatever program you are using.
CathayBrat, I presume that you uploaded a lot of music from CDs and other locations into your iTunes collection. If so, whether you can get these back or not will depend on the condition of your old hard drive or if you backed up.
iTunes stores all its music in C:\Users\USERNAME\Music\iTunes so it is possible to copy the music from that location on your old hard drive to the same location on your new hard drive. If the old hard drive works but replaced because it was full, then you could load from that and back up to other media then load from new drive and restore the music files.
If the hard drive was replaced because it died then the news is not so good. It should, however, be possible to re-download music purchased through iTunes regardless.
The following links have some information and a google search will show more information. Please note that you will not need to download additional software etc to do this so avoid any sites that state that you should download additional software.
iTunes: How to move your music to a new computer
How to Transfer Your iTunes Collection from One Computer to Another