1.) Undo and Redo both power and data cables at both ends.
2.) Temporarily, try undoing the connections to a secondary drive device e.g. an optical drive and using those to connect your 'missing' HD.
You will need to check the jumpers on the HD unless as a presumed slave to Drive C: (its present position) you make sure it is a slave on the secondary cable to another device.
If it works in any of these positions you will know that the HD itself is OK.