If your only change was a small edit to the css you should try backing up your file then remove the change that you made and try to edit the menu again.
If the menu itself has been edited too much you can have this problem. You should always be able to edit the menu as it is produced by Menu Writer. If you think that the menu should be editable after you revert you changes and still have this problem post back with the page that has the menu on it and a copy of the CSSMenuWriter folder in a zip archive. I will see if I get a different result.