My Passport for Mac broken

I have had a “My Passport for Mac” for a few months, but it stopped working yesterday. When I plug it in, the WB icon adds “My Passport for Mac” (switching from “No Connected Devices”), so it recognizes the device was plugged in. Whenever I click “Backup Now,” however, it says “Backup Disk Not Available.” How do I fix this?

Are you using Mac Time Machine to create backup and what is the current system format of My Passport for Mac drive?