Hi there Stunned2016,
As rgd1101 suggested, you need to clarify if this is a separate drive or a partition.
In case it is another one, you need to double check the connections: SATA & power cables. Also, you can attach it to another port.
In case this is a partition, you need to go to Disk Management and see if it requires some action. If there is some unallocated space, you will need to use a data/partition recovery tool in order to retrieve the data.
Cheers,
D_Know_WD