Please read this:
Go to the station where you booked, Give a written application that you lost the ticket. Remember to quote the date on which you booked, the sectors you booked for, name of the passenger/s and travel date, to the PRS Supervisor. He will check and issue duplicate ticket by collecting some charge
The rules are here:
REFUND ON LOST/ MISPLACED TICKETS...
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No refund is permissible against lost/misplaced tickets. However passenger will be allowed to travel on the reservation already made on confirmed one or RAC .If the loss of confirmed/RAC ticket is reported before the preparation of the reservation chart, duplicate ticket will be issued on the collection of the clerkage charge per passenger. If the loss of confirmed/RAC ticket is reported after the preparation of the reservation chart, duplicate ticket will be issued on the collection of the 50% of the fare . No duplicate ticket will be issued after the preparation of reservation chart in case of RAC ticket . Loss of tickets be reported to railways immediately to prevent fraudulent refunds on lost tickets.
ISSUE OF DUPLICATE TICKET IN LIEU OF TORN/MUTILATED TICKETS :
Duplicate ticket can be issued only in case of torn/mutilated confirmed/RAC tickets.
The duplicate ticket can be issued against torn/mutilated ticket if it is reported to the railway and the passenger can travel on the accommodation reserved by him, by making payment and getting a duplicate ticket as per following table :
A) Before the chart preparation
Under this rule the duplicate ticket (in lieu of lost/misplaced/torn/mutilated/reserved/RAC tickets) will be issued before the preparation of the reservation chart, duplicate ticket will be issued on the collection of the clerkage charge per passenger.If the loss of confirmed/RAC ticket is reported after the preparation of the reservation chart, duplicate ticket will be issued on the collection of the 50% of the fare
Refund on torn/mutilated ticket is admissible on torn/mutilated if its genuineness and authenticity are verified on the basis of of particulars visible on the face of such ticket .subject to the deduction of cancellation charges as per rules, provided that the authenticity of such a ticket is verified at the station on the basis of particulars visible on the face of ticket.
NOTE :
1) No duplicate ticket against lost RAC ticket is permitted after the chart preparation.
2) No duplicate ticket is issued against Wait List lost/torn/mutilated tickets.
REFUND OF DUPLICATE TICKET :
If lost tickets are traced and presented along with the duplicate ticket before the departure of the train, refund is permitted for the duplicate ticket after deduction of 5% of the charges paid subject to a min. of Rs. 20/-. Thereafter, original ticket becomes valid for travel as well as refunds.
Please see full details on indianrail.gov.in under Rules>refund
Good wishes.