My website is down; syntax error

First, stop modifying core files, it’s very likely not them causing the problem, and an incorrect edit could lead to a cascade of failures, as you’re seeing. 🙂

Try downloading WordPress again and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.

(This may, or may not fix the problem, but is at least necessary if you have been modifying core files.)

If that didn’t resolve the issue, try manually resetting your plugins. If that resolves the issue, reactivate each one individually until you find the cause.

If neither of those resolve the issue, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue (theme functions can interfere like plugins).

You’re welcome!