I had the same problem as you after replacing the screen and digitizer. I took the phone apart again and did notice that the connector cable of the SIM card tray was loose as suggested in the previous answer. I had to slide it into/under the place it should connect to, in the photo below the small red box shows how the tray should connect, the tab of mine was out and I had to slide it under the brown thing, hope this helps!
http://www.etradesupply.com/blog/wp-content/uploads/2014/03/15.jpg