how exactly this happened? do you crashed after finish killing the boss or crash before that? berserk can be re-enter to the map even you disconnected. simply go back to the channel you entered and you will be able to continue doing it.
If you have killed the boss and you disconnected, there is nothing we could do. what i can suggest you is that whenever you are doing berserker challenge, please re-client before killing to avoid this from happening.