cannot modify header error

my blog worked just fine the nite b4 but the next morning it came out with this error :-

Warning: Cannot modify header information – headers already sent by (output started at /home/suziesit/public_html/wp-includes/classes.php:749) in /home/suziesit/public_html/wp-includes/pluggable.php on line 391

i swear i didnt touch or modify anything. fyi, i could not even gain access to my wp-admin(dashbord). it just appeared for 2 sec and the screen went blank! could someone please please please help me with this. i’ve been out of touch from my blog for a week now. i need to rectify it badly.