If you no-show for a flight and do not show up later on the same day, I believe the segment is canceled and lost. I had the return of an award ticket last July and was not going to use it because of other plans. I called PLT line and told them I would be a no-show, to update their system, and asked about how I could use it later (i.e. within one year of date of issue, call back with the ticket number). Went back to use it months later and was told it was no longer valid because I was a no-show (apparently the CSR did not cancel it in the system). PLT line could do nothing about it, but Customer Service kindly reinstated the segment for use after I emailed them.