Offer Not Available

I ordered some pearls and necklace on offer at £19.99 when I went to claim the offer after being deducted 19.99 from my email I was told delivery would be £2.99 to be paid at the site were the voucher was to be claimed. However when I eventually got onto the site to claim the voucher the site said the voucher code was invalid - so now I have no pearls and no money either how do I get my money back !!!!