hard drive failed how do I get my purchases back?

It has always been very basic to always maintain a backup of your computer files/docs/pics/music/etc. for this very reason.

Use your backup copy to put everything back.

If for some reason you have failed to maintain a backup ( not good), then you can transfer itunes purchases from an ipod: File>Transfer Purchases

I experienced a similar situation where my hard drive that contained my library failed. Unexpectedly, the failure also corrupted by backup drive.

There is some hope though. If you contact iTunes Store Customer Support and explain the situation, they may provide you with a one-time exception to re-downloading purchases. If they do, they may let you re-download everything that you have ever purchased. Make sure to create a backup of your downloads on at least another hard drive or other media.