You can Remove invalid menu items under “appearance->menu” from the dashboard.
If you want restore the menu item. you need check if the page or post on the trash bin. if you can find them from the trash, then you need restore them back.
If you can not find them in the trash. then you need restore your database backup.
Sometimes, if you delete the user that post the page. you will delete all the post the he posted before. that may cause the invalid menu items problem.