My Seagate GoFlex (3.0) external hard drive, all of a sudden, will not mount

My Seagate GoFlex Desk Adapter USB 3.0, all of a sudden, will not mount on my MacBook Pro. It won't mount on my husband's PC either. The disk utility can't see the drive either. I need the data from this drive. I get the following error message:

I never had a problem before with the external hard drive. (I have an old MacBook Pro with OS X El Capitan version.) Can anyone please advise me as to what I need to do?