Something has went wrong with the data, but I am not sure what - I have never seen this happen before where it has blocked the administrator from the back end as well!
To get around this, you will have to use a FTP program to access the site.
Once you have installed an FTP program, login to your site. Go to the theme file that you modified, e.g. the functions.php file. Now get rid of the code that you added in there. Re-upload the file, and you should be good to go.
If you haven't used FTP before, then see this useful guide: http://www.wpbeginner.com/beginners-guide/how-to-use-ftp-to-upload-files-to-wordpress-for-beginners/
If you are unable to use FTP, then you can ask Wordpress if they can restore the site to an earlier version for you:
Please take a moment to rate my service highly, it is how the site rewards me for my time :)(you can still reply back at any time if you need further assistance after rating my service)Thanks and have a great day!Pete.