|Try using test disk to recover your partition table as it looks as though that may have been corrupted/lost.|
TestDisk is powerful free data recovery software! It was primarily designed to help recover lost partitions and/or make non-booting disks bootable again when these symptoms are caused by faulty software, certain types of viruses or human error (such as accidentally deleting a Partition Table).
I have used it myself several times with very good results.
Knowing the original geometry (partitioning) of the HDD you are attempting to fix will be a distinct advantage.
☺ When everything else fails, read the instructions.