[rohrmann]

The nut on the flex line is either inside the faucet body where the flex line would still be or it has slid down the flex line and is on the line inside the cabinet. There should be a rubber washer that seals the connection at the threaded part of the faucet head, and is available in most big box stores as well as most hardware stores too. This is really a very simple fix, so just get your flashlight out and you will find where everything went.