Help WD External drive my passport

My WD external drive files are gone after plugging it into my Mac. Files were there originally and I was able to access from PC and Mac. However, I just plugged it back into my Mac and now it got formatted somehow and set up a new drive under my mac. Files look like they are still there since memory shows storage. How do I recover those files so they are accessible on my PC and how do I prevent the accidental reformatting from happening again when I connect it to a mac?