Okay, since you followed the instructions (mentioned in your question) you should have a backup of the original fstab named fstab.old.
So let's use a live system to boot and restore this old version.
Inside the live system - let's assume you used the graphical interface to mount the partition, we should find it under
So lets open a terminal with
Ctrl+Alt+t and then copy back the backed up file with
sudo cp /media/ubuntu/(something)/etc/fstab.old /media/ubuntu/(something)/etc/fstab
To avoid typing the full address of the mountpoint - especially this
(something) part - you can use auto-completion via pressing the Tabulator-Key twice.
Hope this helps.