I'm an idiot, and I deleted an entire topic rather than an individual post within that topic. Is the whole thing gone forever?
It may be possible that your site host may have your entire site backed up, and you can get a copy of it. I know of one host that does a backup of each site on their servers and stores the data for close to two weeks. They can pull files (such as a database) for a small fee.
Check with your host to see if they can do that for you.
What's the database? First thing RIGHT NOW you should disable the forum, pull it offline, and get the db locally. It MAY be possible to recover the data, assuming the database is protected from any changes right away (or at least make a copy of it to work on).