The only item associated with Ravenclaw is the Ravenclaw Diadem, which is a magical artifact said to enhance the wearer's intelligence and creativity. It was created by Rowena Ravenclaw herself and was eventually stolen by her daughter, Helena Ravenclaw, who hid it in Albania.
The diadem was eventually found by Lord Voldemort, who used it to create a Horcrux. It was ultimately destroyed by Harry Potter in the Battle of Hogwarts.
Therefore, the Ravenclaw diadem is not considered lost but destroyed.