Can't Login, Get a Long Warning… help?

When I try to login, I get the following warning:

Warning: Cannot modify header information – headers already sent by (output started at /home/content/t/a/j/tajharia/html/wp-includes/general-template.php:372) in /home/content/t/a/j/tajharia/html/wp-includes/pluggable.php on line 890

Since I can’t login, I can’t even begin to guess how I’m supposed to fix it. Also, the blog only shows the first page. I cannot go to older posts at all. All of this started shortly after upgrading a few days ago.

Thanks in advanced for any assistance that may be offered.
-Tajharia