This is a feature of ExpressionEngine. You won't be able to use numbers alone as URL Titles. If you managed it in the past, it was probably a bug/fluke that it was allowed, or the entry was created in a non-standard way.
content_edit.php file there's a check for
is_numeric() that triggers an error.
// Is the url_title a pure number? If so we show an error.
$error = lang('url_title_is_numeric');
It's probably in the forums in a few spots, but here's one I found straight from the horse's mouth (though it's just stated as fact, not explained): http://ellislab.com/forums/viewthread/99496/#502754
Edit Looks like Lisa beat me too it.