Missing content

What should I do?

put the line of code back?

or upload an unedited copy of page.php of your theme?

http://codex.wordpress.org/Forum_Welcome#Include_as_Much_Information_as_Possible

If for example you are on about a theme, you make a backup of the current one on your live website and replace it with a copy of the unedited version from the original theme files and see if that resolves the issue.

Example: wp-content/themes/twenthfifteen has a page.php file, if you edited this, make a backup of it somewhere and then download a clean wordpress version, extract it and replace the page.php on your live website with the one you just extracted in the same location.