I think they lock automatically after a week. I really do not know why, it is something the site developer decided upon.
Maybe they lock under other condtions as well. I do not know.
If you want to continue an old thread you can write to a moderator ands ask them to unlock it for you.
You would need to include the address of the thread in your message of course.
I certainly have done that for people.
I am not sure how long a recently unlocked question would stay unlocked for though.