If you got quest to go into Nastrond then you should be able to enter any time thereafter. If it doesn't let you in then contact KI support to address this bug you are having.
I just did this dungeon last night and it was horribly hard. I hope you don't have to do it over - sometimes that is the only alternative. I missed the last stone rose in Dragonspyre the first time fighting Malistaire and had to do it all over again. In any case, the Nastrond trogg is in the final room, very very close to the sleeping giant on the right side. You can tiptoe around there and get it without starting a fight in that room. Good luck!
