WD My Passport Backup Problem?

Hi Just plugged in my WD My Passport external hard-drive. I connected it to Time Machine & it was backing up my whole computer really slowly. Then My computer sent to sleep (I believe this was because the battery was low.) I plugged in the battery & it came back online but said the process was stopped.
I have tried everything I can to get it to start again, but I have no idea how to?
How to I back up my entire computer to this external hard-drive?
I dragged my macintosh HD over to the time Machine Backups & I see it there, but it shows only the few GB of space used when I originally plugged it in have been taken up, so nothing is copied over?
If anyone could help that would be awesome! Thanks

Open System Preferences and click on the Time Machine icon. When preferences open, make sure that Time Machine is set to "on". Next, check the small box that says "Show Time Machine in menu bar". Close System Preferences.

You should now have a small clock like icon in your top menu bar. Click on it and select "back up now". Of course make sure your external drive is attached and mounted. After it does that backup, it will continue to make backups according to schedule.