My Portable hard drive not recognised by mac

My Portable hard drive model WD1600XMS-00 will not open on my mac. It used to. I am running OS X El Capitan Version 10.11.6. Can anybody help please?

Hi there, you can always test the drive on a different computer to rule out factors that may be causing the issue. Check out the link below, it includes troubleshooting steps you can perform for your drive not being recognized on the Mac computer.

thankyou for your response however i still cannot get my mac to recognise
the drive. an old pc could not recognise it either. i do not have the
computer that i first used the drive on.