Find your PUK on the SIM card pack
If you've still got the little pack that the SIM came in, check on the credit-card sized bit of plastic that the SIM snapped out from. The PUK Code is an eight digit number, and is usually printed on the back.
Contacting your network
If you can't find your SIM pack, then you'll need to speak to your network. Your PUK code is tied to the SIM card and the network you're on, so they're the only people who can give it to you. We've listed most mobile networks below - click yours to see how to get your PUK code or the contact info for them.
When you've got the code, follow the instructions on the phone. If it doesn't work, contact your mobile network because entering the wrong PUK ten times will block the SIM card.
You might be asked to set a PIN when you've finished. If you do, make a note of it, because entering it wrong three times will, annoyingly, need the PUK again.
Mobile network contact information