When I click on magento connect manager it opens a page that says 404 Not found 1.
It depends on your sotre but maybe the easy answer is that the downloader was deleted (as a security measure).
Thanks you for your reply! How can I upload the down loader back again? Please confirm. Thanks!
In that case you'll need to download the right version from www.magento.com/download (check on Release Archive). Unzip the file and then upload only the downloader directory to your root magento directory.