A windows PC could write to it and wipe out the formatting. Here's how to start over.
https://discussions.apple.com/docs/DOC-3044 covers the usual.
My roommate plugged our WD passport HD which is formatted for our mac, into his PC without realizing he shouldn't have done that. It didn't show up on the pc and now when we plug it into the mac it doesn't show up. Doesn't show up in System Profiler, or Disk Utility. It lights up indicating it is on when it is plugged in. We tried both usb ports. We tried a different usb cable. Any suggestions? We can't even find it on the computer to do anything for it.