My Network connected External Drive has disappeared from my Mac finder

Hi, for a couple of years I’ve been uploading movies and TV shows to My Book for Mac, a 5TB external drive connected to a WDTV live media player. Suddenly this external disk has stopped being visible, either to my iMacs finder or to disk utility. I know the external drive is working because it plays media just fine. Both the iMac, the WDTV Live player are on the same network. Any thoughts? Thanks
Roy