Having had this problem happen to me today due to flooding, I've discovered the answer buried at the bottom of the NRE page on Advanced Purchase tickets.
If you decide that you no longer want to travel due to the disruption:
Your ticket is non-refundable. However, if the train you purchased a ticket for is cancelled or delayed and as a result you decide not to travel a refund will be offered on completely unused tickets and you will not be charged an administration fee.
If you still want to travel:
If the train you purchased a ticket for is cancelled or is delayed and you still decide to travel, special arrangements will be made to accommodate you on another train (although a seat cannot be guaranteed).
If you no longer want to travel, you just need to go to a ticket office with your ticket and seat reservation for the affected train, and ask for a refund. Depending on the station, it may or may not be easy to arrange, but it is possible. Alternately, you can send in a claim (including the ticket + reservation) as you would've done for a long delay, but this is likely to take longer and result in a refund in rail vouchers.
If you do want to travel, it's generally best to go to the customer services desk and get them to endorse your ticket to indicate the restrictions are lifted. They can also provide advice at the same time about alternate routes. If it's a major problem, you ought to be fine just hopping on another train, but for localised problems it's best to get the ticket endorsed so staff on trains + stations elsewhere know immediately what happened.