FYI, I was having this issue the past couple days. It worked fine for months. Then, the sim card detection started flaking on me, then went out completely. No amount of cleaning the SIM, bending the pins, or compressing the sim card with slips of paper helped.

I went to my carrier store (T-Mobile US), and got a fresh SIM card in order to troubleshoot this issue. The new SIM worked just fine. No clue why the old one stopped working, but too lazy to diagnose further.