Mac doesn't recognize external My Book

Hello,

I was on an old OS for a while (10.5 I think?). I was having many issues in general, but then suddenly my time machine backup stopped working. I use a WD My Book external hard drive, with plenty of space. I can look it up via "about this Mac" and see it as a USB. However, it's not showing up in Time Machine and I cannot in any way select it anywhere to even open the disk. I have upgraded OS to Snow Leopard (10.6.8) and nothing has changed. I have tried unplugging from the computer and plugging back in, still is not recognized. Power is on for the hard drive. Help please!