My SIM toolkit suddenly disappeared and after try a number of options, this is what worked for me on an SGII, Android 4.2.2
Enable SIM lock - (make sure you know the default SIM lock code before you do this, check with your operator)
Apparently on some phones it can reappear by enabling flight mode and disabling it but that didn't work for me.
Hope it helps. Let me know if you found any other ways of getting it back.