The standard themes don't have a "delete" option for gldefault blocks.
To get the block back, use the following SQL request:
INSERT INTO gl_blocks (bid, name, type, title, tid, blockorder, content, rdfurl, rdfupdated, onleft, phpblockfn, group_id, owner_id, perm_owner, perm_group, perm_members, perm_anon) VALUES (3, 'section_block', 'gldefault', 'Topics', 'all', 0, '', '', '0000-00-00 00:00:00', 1, '', 1, 2, 3, 3, 2, 2);
You may need to replace "gl_blocks" with the actual name of your blocks table, in case you changed the prefix.
bye, Dirk