If you have broken your SIM: First, make sure it really is broken. What does the cell phone say? If you see a message like “SIM error,” “Insert SIM”, "SIM not ready” or something similar, try taking the SIM out then put it back in and power up your phone. If reinserting the SIM does not solve the problem, go to your wireless service provider to get a new SIM.
If you have lost your SIM: You don’t want anyone to find and use it fraudulently. Call your service provider and tell them that you lost your phone. They can block it from being used.