My external hard drive suddenly isn't recognized by my Mac?

I've been using my external hd exclusively for backing up my data using Time Machine on my iMac. I'm not sure why, but when I turn on the drive, the Mac no longer recognizes it at all. I've switched out cords, used different USB ports, updated the Debug Kits via Mac's developer site and run the USB prober, but it does not work. I also reset the NVRAM/PRAM. It is not showing up in the disk utility or anything. Anyone have any ideas or insight?