Several Major Issues Right Now

I own three websites. I’m getting different errors on each one.

Right now at https://www.popvinyls.com, my site isn’t loading and I’m getting this error message “Warning: Cannot modify header information – headers already sent by (output started at /srv/disk9/666048/www/popvinyls.com/index.php:1) in /srv/disk9/666048/www/popvinyls.com/wp-includes/pluggable.php on line 876”

I got the same message at my 2nd site https://www.jedikingdom.com yesterday and I reinstalled 3.51 and it corrected the problem. However this is not working for my https://www.popvinyls.com site. When I try to reinstall 3.51, I get this error message:

“Warning: copy(/srv/disk9/666048/www/popvinyls.com/wp-content/upgrade/wordpress-3.tmp/wordpress/wp-admin/includes/update-core.php) [function.copy]: failed to open stream: No such file or directory in /srv/disk9/666048/www/popvinyls.com/wp-admin/includes/class-wp-filesystem-direct.php on line 200

Could not copy files.

Installation Failed”

Any ideas on where to even begin. Is this a hosting issue? 3.51 issue? I’m at a loss.