Wp-Admin error, worked fine – suddenly doesn’t

I went to login to my wp-admin to update my site, but I got this error

Warning: Cannot modify header information – headers already sent by (output started at /home/content/j/e/n/jensenonline/html/wp-settings.php:305) in /home/content/j/e/n/jensenonline/html/wp-includes/pluggable.php on line 391

Not entirely certain what happened, since just a few weeks ago it was working fine and I haven’t updated or messed with anything since. Any help would be appreciated.