External Seagate harddrive suddenly not mounting nor responding to Diskutil

If you have no backup, I'd try a different cable, different socket &/or USB adaptor & even a different external drive case before admitting defeat.

You usually can't get SMART data from a USB drive, but there's an app called DriveDx which can get it, so might be worth a look. (I've personally never used it so no recommendation either way)