I have had to reload itunes onto a new hard drive however my library has vanished. How do I get my library back?

You download it from the old computer backup. The library only exists on the computer you have it on. It doesn't exist anywhere else. If it is all purchased music, you will be able to download all of that music from the store again if necessary.

Glad you got it sorted, however it also demonstrates the need to have a current backup of your computer. That would have gotten you back in business rather quickly. With a new hard drive, hopefully you will be religious about backups, not only for your iPhone, but for the computer as well. Take care.

See Backup your iTunes for Windows library with SyncToy for a suggested backup method.

tt2