Welcome to the forum!
Have you replied to the railway at all yet?
If not, then they may still be waiting for you to make a late reply enclosing a copy of your railcard (which, of course, you can't do as you haven't got one).
So you could write to them / send them an email to confirm that you don't actually have a railcard. That will provide them with the information they need to move forward - and it will also show that you are co-operating with them. It would also be sensible to follow the advice that
@Hadders gives when someone has received a letter from the railway - you can find it in plenty of other threads but essentially you need to persuade the railway that you know that fare dodging is wrong and you won't do it again. If they're certain that you will pay the right fare in future, then they will probably be happy to settle matters by you paying the train fare you should have paid, plus the costs (maybe £150) that the railway has incurred.
There is one other point you need to know - and that is that the railway may well go through your previous tickets. They may be doing that now, which would be one reason why it's taking them some time to write back to you - but if they find that you have bought a ticket when claiming to have a railcard on other occasions then they may well want you to pay back the costs of those tickets as well.