Sky will only have rights for a certain period of time. No doubt it will return at some point.
There is always an indication for when a show/film will leave Now TVs on demand service if is within a 3 month period. This can be shown on the episodes description.
If you wish you can contact Now TV to see if they know when it will return.
1. NOW TV account & payments
2. Cancelling or renewing NOW TV Passes
3. Drop down 'How to get in touch'
4. Live Chat