Lost Item on return

I returned an Item for exchange,as I was sent the wrong size item, the standard response time with regards to returns, is between 3-5 days as stipulated on the Superbalist website. I had not received any response from Superbalist in 8 days and thus emailed them querying my return. They promptly responded, and said that my item had arrived at the warehouse, but it was just a box and contained nothing on the inside. On return my box was sealed with tape, for security reasons. Superbalist has been "investigating into the matter", for over two weeks and I am no longer receiving any responses for my queries. I would like for my item to be either exchanged, or a refunded.