The Barbarian Arena in Svargrond allows access to past trophy rooms you have completed by... doing the next challenge eg. you can access the bronze trophy (Greenhorn) room again after you have completed the silver trophy (Scrapper) challenge and collect any missed trophies. If you missed the gold trophy (Warlord) then you will have to wait till the infamous fourth challenge room is entered, whether from a secret quest being discovered or finished by CiP
The silver Lightbearer trophy requires entry into the Lightbearer reward room in Kazordoon, so if you missed picking it up and it didnt show up in the reward chest in the Adventurers Guild after the Event had finished and room closed its almost certainly gone. To get it you would need to fufill the requirements to win the trophy again and so the first is most likely wiped. Same with the gold Lightbearer trophy.