If you left Sky and then returned after being away for a year then your account will have been closed so you will need to set up a new Sky ID and use:
https://www.sky.com/help/diagnostics/sky-id/sky-id
If you want to use the same email address then you need to first remove it from the old account first as this will then free it up allowing it to be used when setting your new ID up. If you can't do this then you will need to either provide a different email address when setting it up or contact Sky and ask them to remove it from the closed account.