After creating the next season in your team, by entering the tab diary players squad you'll notice the lack of a player list. This situation is intentional and completely normal. Thanks to the player copy from the previous season function, in the new season you will only have players who actually stayed on your team in the new season.
To start copying players from the previous season, please click on the bold text "copy players" as in the picture above or click on the left menu in diary players copying of players.
In this view you are able to select a specific group of players to be shown in your diary for the new season. If a player has left your team or club during the inter-season break, you simply do not mark him, thus skipping him.