Help!! Warning: Cannot modify header information

Have you modified anything recently? If you have, that’s where I’d start. It is probably a plugin or your theme.

You should look at this article: http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F

If that doesn’t work:

It could be WordPress files. If that’s the case, you can download WordPress, de-compress the file (unzip it), and upload the contents of the wp-admin folder over the old files.

For plugins, you can go through and rename their folder 1 at a time to try to find the culprit. (I’d just add an “X” to the front of their folder name in wp-contents/plugins.)