This would usually only happen if the process we use to convert [ATTACH] tags to actual attachment and data records is not working, or if you don't have permission to view the attachment. I've been on your forum, and it doesn't appear to be a permission issue as I can view the attachments when I click the link.
Is this affecting all posts in the forum? What changes did you make before yesterday?
My initial suspicion is an add-on so it may be worth disabling all add-ons to see if that brings them back.