Accidentaly Formated my WD 500GB HD,

Hello,
OK here is my problem that I have come across with my WD External HD 500GB, I was reformating my as I had a virus to reinstall XP. I believe when I was reinstalling windows xp, I believe I had my HD hooked up, an I think I formated it. It now brings up the setups stuff on the HD but only says 645MB all used space, anyone know how to fix it or what to do to fix it, so I can have my 500GB HD back? any help would be greatly appreciated.