There are many reasons when HDD is not detected by your PC.
Maybe you should change another port to connect the hdd, maybe the hdd is not enitialized - go to "disk management" and you will be able to enitialize it and it will be detected by PC.
Sometines it can be caused by virus that the master boot record is damaged then your hdd will not be detected.
Well, in fact you can restore files off the not detected drive by some program like this one http://www.icare-recovery.com/
Most of programs available for free download and free evaluation.