Hi there,
I would advise you to reseat the drive or even attach it to another SATA port. Changing both cable will be a good idea as well.
If the issue persists and BIOS can't recognize your drive, then I guess it may have failed. It will not hurt to attach it to another system and see whether it will get recognized. If it is not recognized over there as well(and the data on it is really important), you may need to use the services of a data recovery company.
Hope this will help,
D_Know_WD