After 45 days with no postings, topics disappear from view, but they can be seen by having the software show "all topics."
There is a box on the right side of the pages that show the list of topics. It's under the profile | register | preferences | faq | search links.
The box says "Show topics from last 45 days" and has an arrow button next to it with a "Go" link next to that. The arrow button shows you options on how many days' worth of topics you can choose to see, and the "Go" link takes you to a page that shows you a list of those topics.