- If it's a new drive, back to the shop and get one that works (warranty!).
- If second hand, ask the previous owner.
- If it used to work with you and suddenly doesn't without you doing anything special, most likely it's gone south. Disk Management, as advised by Mark, is a useful diagnostic tool. It could even be you can revive the drive by deleting all partitions and making a new one. Please tell what happens.