If it's a completely new topic, use the create a new thread link on the Forum Menu - but use the Filter or search engine to check to see if your topic already has a thread you can resurrect.
There are some old threads that were closed for housecleaning purposes, particularly old "BS" threads. They can be reopened as needed - just ask.
In some closed threads, it's obvious that they were closed because of contentiousness or obnoxiousness or repetition - generally, we don't reopen those.
-Joe Offer-