bbPress.org

Sounds like you modified a file incorrectly. Without seeing your template files, it’s going to be hard to guess what you did wrong. You can try validating the XHTML of a page here and maybe it will point to a problem.

http://validator.w3.org/

You can also use a text editor with syntax highlighting and you should be able to see the error, maybe an unclosed tag or something like that.

Finally, you can check the differences between the stock theme and your modified files and see if you can spot the difference.