How long have you had the forum?, and how long have you had the problem?
Have you done any additional plugins, wordpress, bbpress or other plugin upgrades, or changed css or functions recently?
What versions of wordpress and bbpress are you on?
The forum was just a few days old when it stopped working and only had a few forums and topics. It’s on a new website that I’ve been working on. All my software and plugins are the latest version I have the following plugins.
Akismet
bbPress
Category Reminder
Jetpack
Limit Login Attempts
myRepono Backup Plugin
New User Approve
Post Expirator
Simple Notices
System Snapshot Report
TinyMCE Advanced
TinyMCE Spellcheck
Updater
WangGuard
WP Display Header
WP Super Cache
Ok, I’d try deactivating all the plugins (except bbPress!) and see if it then works.
If it doesn’t work with all the plugins disabled, switch to default theme such as twentyten and see if it then works.
If it works with plugins disabled, add back the plugins one at a time and see which one is causing the conflict.