Sounds like a problem with the code found in the functions.php file of the theme you are currently using.
Fastest and easiest way to deal with it is to connect to your site via FTP and rename your active theme folder:
Rename it to:
This will cause WordPress to de-activate your active theme and the error message will be gone. You should then be able to login to your Dashboard by going to http://yourdomain.com/wp-login.php
Then activate a different theme (like the Twenty Ten or the Twenty Eleven theme) - which will give you a chance to resolve the issues in the functions.php file of the theme you were using.
Once you've resolved the issues in your old theme, you can rename the theme folder back to what it was originally and try re-activating it again.
Hope that helps.