We have booked a ticket online from IRCTC for five members. 1st has cancelled his ticket. But due to unavoidable situation last 2 members have to cancel their tickets. Is it possible that the 1st member's ticket will be reinstalled?
The technical answer to your question is no, tickets booked are only valid for the passenger and cannot be restored once cancelled. However as @jpatokal rightly pointed out change of name is permitted under special circumstances. Change of name within a family or government officials travelling can use this facility.
Practically though, most ticket checking staff would only check the identity card of one of the travellers, so you may get away with changing one of the other travellers, though this is not legally permissible.