Hi
more information required
Do you mean hard drive missing or several partitions on one hard drive and partition missing ?
Is C: working normally but D: not readable or both have problems ?
If 2 physical hard drives check data cable connections to hard drive at both ends (drive & mother board) and power cable at hard drive.
What drives show up in device manager. ?
(should show if one or two hard drives and what make / model)
regards
Mike Barnes