Try turning the phone off and taking out the SIM Card and then putting it back in, it may not be seated properly in the SIM tray.
Also make sure your phone isn't set to 2g or gsm only in the settings menu under networks.
Power off your phone, remove the sim, put it back in and start up the phone. If that doesnt work, google a imei checker and enter your imei to see if your phone is blacklisted. If not blacklisted, head over to a koodo store for a sim replacement.