You come out to get in your car in the morning and find something important is missing, and in this case it isn't the catalytic converter. It's the number plates. What to do?
Get to the nearest police station as soon as possible and report the theft, get a crime number. But it doesn't end there. Suppose the thief is intending to use it for an illegal sale or a crime? You need to be able to prove where you were if such a thing occurred and your number plates were used, because by now you will have replaced them and there are now two cars with the same number plates.
Take a photo of your car with some evidene of the date, e.g. a newspaper, as soon as the new plates are on it, then, I am afraid, keep a meticulous log of your movements in case you need to challenge speeding tickets, parking tickets etc caused by the thief.