You might got some problem with the master boot record and maybe the partition table is gone too. And that's why you see your partition as unallocated space. You partition is lost.
To retrieve lost data, here is some tutorial for you http://www.icare-recovery.com/partition-recovery.html
Advice: not touch the drive which is wrong until you restore the files.
After that, you may go to "Disk management" to create partitions.