If for some reason your anonymous user can't be loaded anymore, this script should help, if applied as per Executing fix scripts.
<%@ page contentType="text/plain" %>
<%@ page import="info.magnolia.cms.module.ModuleUtil"%>
<%@ page import="javax.jcr.ImportUUIDBehavior"%>
<%
ModuleUtil.bootstrap(new String[]{"/mgnl-bootstrap/core/users.system.anonymous.xml"}, true, ImportUUIDBehavior.IMPORT_UUID_COLLISION_REPLACE_EXISTING);
%>