How do you unlock your Sim card? Why does my phone say "Invalid Sim Card"?

[AC Question]

I turned my phone on at school and I had a notification that said "Invalid Sim Card" I took off my phone case and took out the battery, the Sim card was still there. So I put back in the battery and turn it on but it still said "Invalid Sim Card" What should I do?

 

[hallux]

Contact your carrier. Your SIM may have become corrupt.

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