I was told by the customer support that you have to be able to prove that you own the phone number (and sim card) in order to report loss (otherwise, anyone can report loss on other's sim card), for instance, providung the information about your last refill (amount, date, etc).
Once they believe your information, you pay $25.00 (?) to purchase a new sim card, they will cancel your old sim card and transfer the credit to your new sim card.
I keep the receipts for all my refills.
You can confirm this with the customer support yourself.