Well, the first thing to do is to check all your cable's and connections to the drive. Be sure the power is properly connected and turned on. Shutdown the computer and wait 30 seconds. Connect the drive to the computer. If you drive supports USB then use a USB cable, otherwise go with your your Thunderbolt setup. Power up the computer. Does the disk icon show up? If not then open Disk Utility and look in the sidebar. Do you see a listing for the drive? If you don't, then there's a problem, but what, exactly, is hard to know. It could be:
- The drive has failed.
- There is a problem in the enclosure.
- No power is going to the enclosure or drive.
- You have a defective cable.
- You have a bad port on the computer.
Any of the above are possible. I would take your computer and the drive in for a diagnosis to your local Apple retailer.
Now, if you do get the drive to show up, then I would use Disk Utility to repartition and re-format the drive:
Drive Partition and Format
1. Open Disk Utility in your Utilities folder.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Apply button and wait until the process has completed.
4. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.
5. Set the format type to Mac OS Extended (Journaled.) Click on the Security button, check the button for Zero Data and click on OK to return to the Erase window.
6. Click on the Erase button. The format process can take up to several hours depending upon the drive size.