Rejected SIM cards are quite easy to fix, firstly you’ll need to locate the SIM card on the phone. Generally the card is stored under the battery. You’ll probably have to remove the back cover before you can slide out the battery. Be sure the phone is turned off before doing this.
Remove the battery and look for the SIM card. It is usually white or yellow with circuitry printed on it. Sometimes it has text on it as well. The card may be as small as a thumbnail.
Blow around the area where the SIM card had been lodged to disperse any dirt or dust.
Contact your cell phone service provider if the SIM card is still rejected. You may need to replace it.