External Hard Drive suddenly not recognized

I have a WD 500GB external hard drive that I use to back up my data. I only have one hard drive as I'm currently living abroad and couldn't bring a 2nd with me.
I plugged in my external last night to back up my MacBook Pro and it backed everything up. I even went into Time Machine to go back and restore a file or two and that worked without problem. I have been using this drive for about a year to back up everything on my Mac and, although sensitive, it seems to work okay.
I plugged it in this morning and nothing. My Mac will not recognize that the drive is not plugged in in Time Machine or in Finder. If I go to Apple Logo > About This Mac > More Info... > Storage tab, it just shows my internal hard drive and my CD/DVD drive. If I click on Disk Utility with the drive plugged in, it shows my Hard Drive (Macintosh HD) with two icons, one inset below the other and both in black font, and then my external hard drive has two underneath, but only the top one is black font - the one underneath is greyed out.
It goes like this:
250.06 GB Hitachi...
Macintosh HD
500.08 GB WD My Passport
Time Machine Backups (greyed out)
Superdrive (greyed out)
When I clicked on the 500GB one and told it to repair/verify it said the disk was working fine. I tried unplugging and replugging in the drive (it is not recognized in Finder so I could not properly eject the disk). I restarted my Mac with it unplugged and plugged it in afterward and nothing.
The HD itself is working, the light is blinking (not solid) and I can feel the hard drive spinning, but I'm at a loss as to what to do to get my Mac to recognize it again.
Any suggestions would be helpful.
Thank you!

I also wanted to add, when I click on the greyed out Time Machine hard drive that's listed and then click "Info", this is what it says:
Name : Time Machine Backups
Type :           Partition
          Disk Identifier :           disk1s2
          Mount Point :           Not mounted
          File System :           Mac OS Extended (Journaled)
          Connection Bus :           USB
          Device Tree :           IODeviceTree:/PCI0@0/EHC2@6,1
          Writable :           Yes
          Universal Unique Identifier :           56EF6088-FFCE-300B-81D7-3E0986532AB5
          Capacity :           499.74 GB (499,735,552,000 Bytes)
          Owners Enabled :           No
          Can Turn Owners Off :           Yes
          Can Be Formatted :           Yes
          Bootable :           Yes
          Supports Journaling :           Yes
          Journaled :           No
          Disk Number :           1
          Partition Number :           2

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