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.
I disabled all add-ons and the issue still remains. I have not done any changes to the forum and it's only on some sections of the forum not all the forums. I checked the permissions and they look good to me. For example, I posted this yesterday and when I submitted it, it was fine. Today, it has the text instead of the actual image http://www.collectorsunleashed.com/forum/threads/my-new-car.3195/
I see what is going on now. It seems that these posts are not showing the images once they are also added to the article add-on created by @wang
If I disable the add-on, the attachment links do not work and go to a "you don't have permissions error page". If it is enabled, you can click on the attachment link but it's just a link, not a picture showing as I wanted it.
It sounds like the add-on might be doing something to the attachment records when they are added to that add-on. For example, if it switches the attachment content_type and content_id to something else then it could cause this.
At this point it might be worth continuing the troubleshooting with the author of that add-on.