Do you still hear the drive spin up normally??
If you do, congratulations, you are the proud recipients of the infamous Seagate HD bios failure that killed thousands of HD's when the boot bios on the HD goes into a forever loop and is no longer recognizes or boots.
This is why I no longer buy Seagate drives. Yes your data is there and intact, the drive just can't boot anymore and its Seagates fault due to a bad bios.
They will however offer to restore your data for $2,400.
Cheers.