I am guessing that index.php’s permission was changed such that the website couldn’t overwrite that file. The solution probably is to check your index.php file permissions and ensure that whatever web server you use can actually write to it.
1) index.php’s permission could’ve been changed such that the webserver user(the account that the webserver runs on) couldn’t write to it.
2) the permission to the parent directory of index.php could’ve changed.
Since you haven’t specified where this index.php is, I’m going to assume it’s part of a theme.
Ensure your theme directory (and its contents) is owned by the account that the webserver runs under.
Without much details, that’s my answer.
