Welcome to our Community Forum Mickmic and sorry to hear you're having problems with your SIM card.
So we can help, can you give us more information such as what phone model you're using and what code you was entering? For example is it the PIN code, a PUK code or the phone unlock code?
Please also tell us where you purchased the phone?
I'm sorry for the questions but the more information we have the better we can help you.
Thanks,
Lisa