[r-a-y]

The good news is the thread isn’t entirely deleted!
The bad news is you’ll have to go into your DB to retrieve it.

Login to PHPMyAdmin or whatever is your SQL tool of choice and navigate to wp_bp_bb_topics (or whatever is your bbPress table).
Find the topic you want to retrieve, then edit it and change the “topic_status” column to “0”.

Now your topic should be rescued (hopefully).