Hi @Ann04,
Welcome to the Three Community.
Most people don't use SIM pins these days, it's an older security feature where you would enter the pin when switching your phone on, these have largely been replaced by device security features such as phone pin/pattern/fingerprint unlock etc, as you can leave your phone on and still secure with these.
Perhaps it was activated in your settings by mistake. The default codes tend to be 0000 or 1234, or if all else fails you should be prompted for the PUK code if you enter a SIM pin incorrectly 3 times. Customer services can let you know the PUK for your SIM.
The alternative would be to get your SIM swapped, the team can either post one or you can get one issued in store if you bring photo ID.
Hope this helps!
Jonathan
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