Millions of pounds a year are lost to the rail industry because of ticket avoidance and abuse. Unfortunately to deal with this measures have to be put in place which can penalise people who have made honest mistakes. In most cases, certainly in the case of a forgotten season ticket, the TOCs customer services will rectify any genuine issues.
This does not remove the requirement to have a valid ticket for your journey.
Incidentally it is common practice to notify onboard staff of out of course ticket office closures and also inoperative ticket machines.