phone (4s) was working fine--now a notice that there is no sim card in phone....what happened, what to do next?

Take the sim card out and reseat it and see if that helps. IF not place another known working simm card into the phone and if that works then the problem is with your sim card

First thing to try is to reset your device. Press and hold the Home and Sleep buttons simultaneously ignoring the red slider should one appear until the Apple logo appears. Let go of the buttons and let the device restart. See if that fixes your problem.