Oh no! Hard drive failures and data loss are just about the worst things ever. Believe us, we commiserate with you and want you to get your music back!
For assistance with this problem, you're gonna want to contact Bandcamp (the site that hosts our music for us) directly. Check out Bandcamp's help page. Their support email is right at the bottom, and they can help reauthorize a download so you can get your music back.