If your SIM is not working, the first thing to do is see what the cell phone says. 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. Typically the SIM is underneath the battery, but it’s best to check with your cell phone manufacturer for instructions.  For an example, the link below shows you how to remove a SIM in a Nokia cell phone.
In most cases, the phone has just lost its connection with the SIM (maybe because the phone was dropped) and reinserting the SIM will solve the problem.

If your phone got wet, let your phone dry out before you start it.  The SIM itself will not be damaged by liquid; it also resists shocks and a high range of temperatures.

If reinserting the SIM does not solve the problem, go to your wireless service provider to get a new SIM.