1) You don't need the RewriteCond line. It's intended to determine WHEN your rule applies, not WHICH PART of the request.
[EDIT] UNLESS, your RewriteCond is there to make this rule apply whenever the query string is empty. If that's it, then there's nothing wrong with it.
2) I think you need to include the first / in your rule, like this:
RewriteRule ^/([^/]+)$ /somepage.php?id=$1 [L]
[EDIT] Both your version and mine will match your index.php file, which might explain why your site is broken right now. If you don't want to match php files, you could add another condition like
RewriteCond ${REQUEST_URI} !^.*\.php$
3) For a rule like this, it might be helpful to add /?
at the end (outside the parens, before the $), in case they look for foot.com/xxx/, which makes sense if you want it to look like a directory.