please look at your configuration file. There should be an item:
`
$conf['special_pages'] = array(
'page'=> array('define'=>'_PAGE_', 'rewrite'=>'page'),
'search'=> array('define'=>'_SEARCH_', 'rewrite'=>'page/search'),
'archive'=> array('define'=>'_ARCHIVE_', 'rewrite'=>'page/archive'),
'tags'=> array('define'=>'_TAGS_', 'rewrite'=>'page/search/tags'),
'news'=> array('define'=>'_NEWS_', 'rewrite'=>'news'),
'category'=> array('define'=>'_CATEGORY_', 'rewrite'=>'news/category'),
'news_archive'=>array('define'=>'_NEWS_ARCHIVE_', 'rewrite'=>'news/archive'),
'pages'=> array('define'=>'_PAGES_', 'rewrite'=>'pages')
);
`
in it. If not then you will need to restore the contents. Depending on what caused the initial problem you may be able to restore these with the rewriteTokens plugin. If not you will need to delete your config file and re-run setup. Be sure, of course, to make note of your database setup first.