Interesting, how much credit did you have left? I assume it used all of your credit, which is why it said insufficient funds, but I still wonder how much credit you had left because when my little brother gets members by SMS it just deducts from the value of his credit and not the actual amount of money he put on his SIM.
If you are on prepaid I assume the money is gone, unless the SMS they sent you saying you didn't have enough credit said more that you didn't mention in the post. Check your phone's balance to see if you still have your cash or not.