The official policy is that you will have to buy a ticket for the next available train at whatever bucket it's currently priced at (usually pretty close to the highest bucket for same day travel).

That said, I have been able to talk my way into exchanging my ticket for a later/earlier train and not have to pay any difference.

Either way, you'll be able to get a complete refund on the missed train if you haven't printed the ticket, or if you have printed it, use it towards the next train's fare.