I found that my issue was that there is a piece of metal at the bottom of the SIM card slot that when you put it the sim it connects with a sensor that tells the phone a sim is inserted. But the issue is the metal cage does not cover the full sim area and the metal arm can dislodge at the bottom and doesn't connect. I had to take the back off and push the arm back in alignment to have it recognize the sim again.
Hope this helps,