Due to some weird Windows bug (or maybe a naughty malware that's yet to be detectable by numerous scanners), you may find that your Windows machine no longer automatically assigns a drive letter to any external hard drive you attach to your PC's USB ports.
Strangely, USB flash drives work perfectly fine.
A simple check in Device Manager will show you that Windows did in fact see the hardware. Disk Management shows the partitions, too, except without a drive letter. You could just assign one manually from inside Disk Management but why should Windows not do it's job?
Here's how to get it working again. You'll need to run this from Command Prompt.
DISKPART> automount enable
That should fix it. You don't even need to reboot.