All closed Topics can be re-opened by the person who created the Topic in the first place. Topic participants can access the conversation at any time, even if the Topic is closed.
However, if you want to continue the Topic conversation, there's no need to create a new Topic. Just re-open it and you can continue your conversation from where you left off.
To re-open a Topic, select it from the Topics module. It will open in the left panel. Then click on the More options button in the header and press Re-open. Remember, if you can't see this Re-open option, it means you didn't create the Topic. So contact the Topic author and ask them to reopen it for you.