My Book not detected on mac

My ecternal My Book 3TB was working fine previously until yesterday when i tried to connect it, it wasn’t detected. I’ve tried to restart my mac (thought it might be mac’s prob). My mac is still Mac OS X 10.9.5. Can anyone help because I need the files in there in urgent!! Thanks a lot!!

Welcome to the Community.

Have you tried changing the USB cable or looking for clues in Disk Utility?