Something has either prevented the javascript that runs the editor from loading or from running, or the associated css from loading, or it may be a php issue that then causes the javascript issue. This could be a result of something else you have changed or added to the page, a theme or addon, or a template override, or just a missed file when rendering the page (this often happens on WAMP development systems).

Some things to check:
- Clear the concrete5 cache and refresh your browser cache. It could just be a file that got skipped by your browser.

- Use the developer console or firebug to check for script errors (console tab) and for incomplete requests (record on the network tab). (seehttp://www.concrete5.org/documentation/how-tos/editors/getting-help... )

- Check the php error log file for php errors

- Check the concrete5 log for php errors.