You will need to contact your host for assistance if you no longer have the merchant key that works. If they are a ShopSite partner then they can contact ShopSite for assistance. If they are not a ShopSite partner you will need to purchase a support incident at:
http://shopsite.com/support_pricing.html
Note that you will not be able to view any credit card data for orders placed with your old key once you have created a new key. So I would check your backups and try all key that you might have created previously (only the last one should work) before you take the step of having the encryption method changed by your host or ShopSite.