A workmate and me experienced the same problem just yesterday. Our hard drive is the portable one (usb 3.0). It is hard to think about a coincidence since we have found a couple of posts in the last 48 hours describing the same problem.
The drive was not suddenly disconnected. It is always connected to my laptop and I shut the laptop down as usually. The last time I powered it on, it got stuck reading the drives for about 5 minutes showing a black screen not being able to load Windows. Then I powered the laptop off, disconnected the external drive and powered the laptop on again.
Then Windows boot normally, then I plugged in the external drive. Windows Explorer doesn’t display any partitions, but the device manager does as not initialised, unpartitioned and all space as available. Actually it had 3 NTFS partitions and was almost with no available space. I guess that the partition table or the MBR is dead…
I have checked the drive with all TestDisk, MiniTool Partition Wizard and MiniTool Power Data Recovery but they does not find any file. I will finally check it with GParted when I have some free time.