login via cpanel or whichever CP you have, go to the file manager and rename your current theme directory ( example currenttheme/ ) to something else like 'aaaaa/' ( you can do the same via FTP )
now you should be able to access the admin area /wp-admin/
if you don't know how to comment out a snipped from a php file I'm assuming your theme has not been edited or customized so simply reupload the original theme and you should have it working again. ( upload it into a directory named currenttheme/ )
In case you have never had this theme working simply install another, it means there something not working in the theme.
This is just a quick way around the problem, fixing it would require you to know at least some basic php probably.