If your phone is asking for a PUK code, it means your SIM has been blocked by your network.
If you don’t already have it, you can acquire a PUK code from your network either online or by calling customer support. All the current UK network Customer Helpline numbers are here.
Once you have your PUK code, key it into your device when prompted (it’s typically the only thing you can do on a device with a blocked SIM).
You get 10 attempts to input your PUK code, which should be plenty, but be aware that inputting it incorrectly 10 times will permanently disable your SIM and you’ll have to order a replacement.
When the PUK is input correctly, your SIM will be immediately unblocked and you will regain access to all of the features on your device.