usb external HDD to get wiped & Unreadable !

usb external HDD got wiped & Unreadable !
was caused by loose connection in my digital set top box —
its a 930 gig HDD I hade family photos & videos – 610gig of all gone

computer disk management – says 930gig Unallocated . so this means its Unrecoverable , right ??
Also the drive letter does not show anymore in Explorer- do I have to reformat the Harddrive ?? or is it possible to still recover those 610gig family photo & video ?? and How ??