If your Season ticket has expired, it will not cover your journey in full. If it has been left behind, or is for Standard Class accommodation but you wish to travel First Class, you must buy the appropriate ticket before you travel.
If you are travelling from a penalty fare station on a penalty fare train and your Season ticket does not cover you for all of your journey, you must buy an accompanying ticket or permit to travel, if you are able to*, before you start your journey. If you do not, you may be liable to pay a penalty fare.
More information about Penalty Fares