Cannot Login to wp-admin. Warning: Cannot modify header information…

Yesterday I was trying to add drop-down menus to my header and I installed the WordPress CSS Drop-Down menu plugin. I added the required code to the header.php and function.php files and set up my widgets. The result was not what I expected, so I removed the widget from the header.

The next time I tried to login, I got this warning:

Warning: Cannot modify header information – headers already sent by (output started at /home/swimming/public_html/wp-content/themes/SilkyBlue/functions.php:16) in /home/swimming/public_html/wp-includes/pluggable.php on line 890

There is nothing else on the page.

I went into cpanel and deleted the plugin code from the header.php and function.php files but nothing changed. I still cannot login.

Any clues as to how to proceed?

Kat