Yes, threads can be unlocked. However, this is available only to the Editors and the Webmasters. So, to continue the discussion further, we can create a new thread and give reference to that old locked thread in the current thread.
Threads can also be locked when it becomes much older. Whenever, a new message is posted on a particular thread, that particular thread appears at the top of the list in the Forum section. Hence, the new threads get further down the list and might not fall into the attention of the members. Thus, an important thread which may be important or requires special answers may remain unattended or unanswered. So, it is good to lock up a thread when it becomes old or the appropriate answers are received as mentioned by Andy.