If its an old thread (and I would say that normally means more than a year old) and its another question on the same topic, then I would prefer to see a new thread created with the initial post including a link to the original thread. That way someone reading the thread knows its a new thread, can still see the original discussion for means of background, and is not confused by any old content or misunderstanding the dates and starting to re-respond to old information.
Reviving an old thread wit a new post should really only be done for the purpose of providing follow-up information for the original discussion (i.e. an outcome).