Hello. My name is Michael. I can help you with your question.
the PUK isn't something that you would set. It is something that is assigned to the SIM card. If there is a problem accessing the SIM card a certain number of times, it will lock the SIM and you will need this code to unlock it. If you have the original card that came with the SIM, that should have the PUK on it. If you don't you can still usually get it from the cell phone service provider by contacting them and providing them with the information that is on the SIM card. It may be just as easy to have them replace the SIM with a new one - since you mentioned there was nothing unusual going on, it shouldn't just lock like this on it's own... it's usually only if someone assigns a PIN to their SIM card (very few people do) and then someone else tries to unlock it too many times.
Please let me know if you have any questions.