Your SIM card reader may have been displaced with the fall, turn off your device and open your phone, then remove the card and put it back in. If this does not solve the problem make sure that you are receiving good reception as this may be the problem all along. If not any of these steps seem to fix the problem then your SIM card reader may have broken with the fall and you'd need to replace it. Visit the troubleshooting page for more information: [[Sony Xperia J Troubleshooting|Troubleshooting]]