Disconnect and reconnect, try in another USB port, try in a diff computer, unplug all other USB devices, check what device manager detects it as, file a post on the Western Digital portable drives support forum.
Just FYI, try not to leave external drives plugged in all the time. Keep them somewhere safe and use them for backups or transfers. If you need a regular use drive, get an internal. I have had so many WD ext USB failures recently it's a bad scene.