You can't use the same login as you did before as that will have been closed after you left so logout of the app (via settings) as you will need to set up a new Sky ID to use with it.
If you want to use the same email address then you need to first remove it from the old account first so log into the old account here:
https://www.sky.com/myaccount/my-details
Then delete the email address by providing another one and logout of this account, this will then free it up to use when setting up your new Sky ID:
https://www.sky.com/help/diagnostics/sky-id/sky-id
Then use the new ID to login to the app with.
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