My HTC First will not recognize SIM card

Hello. I have an HTC First Facebook phone that will not recognize my SIM card. I recently got an unlock code from ATT for it and they said to put a non-att sim in it. I did that, but it didn’t ask for an unlock code. I don’t think it’s a SIM card reader problem because it was able to recognize my phone number but that’s it. Keep in mind I used a nano-sim to micro-sim adapter because the phone didn’t support nano-sim.