Even if you have a reservation and miss the train, your ticket is not useless. In that case, you can file a TDR i.e. Ticket Deposit Receipt. By filing a TDR, you can claim a refund for your ticket from the railways. For this, you have to fill the TDR online within 4 hours of the train's departure.
You can complete this process through the IRCTC website or app. If you file the TDR within the stipulated time frame, the railways will refund your money after some deductions. Keep in mind that this facility is not available to those with general tickets. This only applies to reserved tickets. On the other hand, if you have bought your ticket from the counter, you have to go there and file the TDR.