WordPress error when using “www” version of my site

I get the following error when I try to bring up my site using “http://www.mysite.com” but when I bring it up using “http://mysite.com” (without the “www”) it works just fine!

Warning: Cannot modify header information – headers already sent by (output started at /home/alldeal1/public_html/homerepairgeek/index.php:15) in /home/alldeal1/public_html/homerepairgeek/wp-includes/pluggable.php on line 770

This just started happening, and I have no idea why. The www version worked before. I searched fixes, and I have verified any extra white spaces are not there, at least in the index.php file. Thanks to any who can help!