I can see my external hard disk information under the Control Panel/Device Manager/Disks but when I look at Windows explorer or My Computer, it does not show there. There is a lot of important information on the external hard drive and can that be recovered?
I'm going to need a bit more information as to the brand and the type of Hard Drive that it is (SATA, SSD, SCSI) as well as what OS you're using.
It sounds like an unusual occurrence but probably has a simple solution. Something that usually works is restarting the PC. I have no idea how many times I've been saved by just restarting. I always do everything else before I restart it yet it fixes the problem 50% of the time.