This usually happens when there is a coding error in a custom block, or 2 addon blocks on the page are incompatible.

You can solve it by:
1. Go to the site map in the dashboard

2. Select the problem page

3. On the popup menu, select versions

4. Select a version from before the error and approve (thumbs up icon) that version

5. Select all versions after the now approved version and delete them.

This howto contains more:
http://www.concrete5.org/documentation/how-tos/editors/the-block-do...

John's answer is the right one if the error has been caused in the CMS itself.

If the templates have been edited, then it won't help you.

Invariably the missing header bar is caused by the footer_required statement being missing from the template.

Although you do say this is only broken on one page, so unless it's a single page John's is almost certainly the approach to take.