Cannot mount/unmount external hard drive on Mac
I just had this happen to me. If it won't repair on Disk Utility, you can try Disk Warrior. You might get lucky, however in my experience since the drive wouldn't mount, it was impossible to get anything. 
I was able to get some data back through a data recovery service, which cost around $220, but I am discovering that some if the data is corrupt and won't restore. 
I would recommend that you take the drive to a professional. Anything you can do might do more harm.
And have a double-backup plan.