This will have happened because you have a mistake in the code that you have entered. To fix this you will need to undo the changes you have made to the functions.php
file. You will have to do this with either ftp access or control panel.
The functions.php
file can be found in the following location: [INSTALLATIONFOLDER/wp-content/themes/THEMENAME/functions.php
Once you have found that file, edit it and undo the changes you have made.