yes, absolutely you should be worried, this is a recipe for food poisoning par excellence. If you are just going to let them stand, then leave them covered in a cool place in a sieve so that the melt water drips away from the prawns. if you want to hurry them up, then put them in a plastic bag in a sink of tap cold water. You can cook them from frozen and serve hot. If you are going to use them IMMEDIATELY, then you can use the defrost setting on your microwave very gently to start the thaw. I would not advise this method for people who would be seriously at risk from a tummy bug or who have a tendency to them. For complete safety, follow the instructions on the packet.