Welcome, beattieboyes!
I'd strongly recommend you go to
Disk Management and Device Manager and see if the external 1 TB HDD is recognized there. Does the drive power up when you connect it, though? You should also try plugging it to a different USB port on your computer and also attempt using a different USB cable. If you have access to another computer, you might also want to try connecting the external there.
@CountMike is right, though, you should give us some more details about your system, so that we can be more helpful.
Keep us posted.
SuperSoph_WD