I accidentally turned my sim pin on and I have no idea what it is. What do I do?

Contact your carrier. They can help you with your SIM PIN.

You'll likely want to disable this. It's a feature that prevents your cellular data from being used after every reboot until you enter the Pin.

More details available here: Use a SIM PIN for your iPhone or iPad - Apple Support

the SIM pin will be asked for when you reboot your phone or possibly when you turn your cellular connection off and on. Not a bad thing to have this turned on and you hardly need to use it.

Just don’t forget the PIN, else you will need help from your provider to unlock your SIM again with a PUK code.