New Hard Drive- Error Message :(

I just got my external hard drive a few days ago, and it worked just fine for the first few days.  Yesterday, though, when I plugged it in, I got a message saying ‘The disk you inserted was not readable by this computer.’

Obviously, it WAS for the first few days.  I’ve never had this problem with an external hard drive before… I any idea why it suddenly won’t work or if I can fix it? 

Thanks :)

Wanted to add- 

In the Disc Utility, it comes up, but it says under S.M.A.R.T. Status, “Not supported” and it wont let me do anything with it. 

Interestingly, the SmartWare icon does come up just fine.