error message

I get the following error messages when I try to login:
Warning: main(./wp-blog-header.php): failed to open stream: No such file or directory in /home/mysite/public_html/wp-admin/index.php on line 3
Warning: main(./wp-blog-header.php): failed to open stream: No such file or directory in /home/mysite/public_html/wp-admin/index.php on line 3
Fatal error: main(): Failed opening required ‘./wp-blog-header.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/mysite/public_html/wp-admin/index.php on line 3
I don’t know what could be missing. I was able to work in wordpress last night without any problem. Please help.