Why my WD Passport for Mac isn't mounting or readable?

Hi.

I got a 500GB WD My Passport for Mac a couple months ago when all the data on my macbook needed to be cleared off. This is where all my old files are stored. I have been able to view all these files as the external harddrive was working great. I haven't used the passport for a couple weeks, but today I wanted to store some new files on it. When I plug it in, the message: "The disk you inserted is not readable by this computer" comes up. If I ignore it, I still don't see it.

Disk Utility tells me the drive is not mounted (not sure what that means). Also, verifying and repairing both fail when I try. Any ideas on how I can get this thing to work again?

I do not want to lose the files stored on the disk.