If there have been no posts on a thread for 30 days, it does get archived, but there are ways to get access to it. If you have an important thread or a post in an old thread that you really need, you can PT a staff member, who might be able to find it for you in the archive.
If you want to keep a PT from archiving, you just need to make sure to bump it by posting something in it once a month.
Threads are archived on Mondays after thirty or more days.
The only way to be able to get information from an archived thread would be to contact an online Moderator or Admin for assistance, and they may be available to assist you in getting your information from there.