If a setting in the control panel has been entered incorrectly it may lock you out of the control panel.  Config variables are stored in the database, and can be difficult to edit directly, however they can be over-ridden by entering the appropriate lines in /system/config.php.  Variables in /system/config.php are read preferentially to database config variables.

This most often happens when mis-setting the cookie value and leaving off the leading period.  You can add this line into /system/config.php to change it:

This should allow you to enter the control panel.  You will want to head to the appropriate setting and submit it in the control panel to force the update to the database.  Once that is done, remove the over-ride from config.php.