Added footnote for stats plug in now cannot get into my blog

HELP!!!!!!!!!!!!!!!!!! I am an absolute beginner and have completely messed up my blog. This is what I did. I loaded the wordpress.com stats as a plug-in which loaded but didn’t count the visits. I then read that I must add <?php wp_footer(); ?> I did this on the last line of the functions.php, footer.php, main index.php and maybe under a number of other places. Somehow it started counting the visits but I cannot get in my site (except from my other blog and then I’m limited to the stats counter). I tried to delete my entries but then could not update and it gave this message: Warning: Cannot modify header information – headers already sent by (output started at /home/thawtsc/public_html/wp-content/themes/sleek-black/functions.php:8) in /home/thawtsc/public_html/wp-includes/pluggable.php on line 865
I now cannot login to my blog as I get the same message