There are several possible problems with your phone: 1) There is no network in your area or the network is down. The only thing to do here is to contact your networks customer support. 2) The SIM card is not plug in properly or the SIM card is broken. To replace the SIM card in you phone follow this guide here(link).

The Galaxy Note 4 has a built in water damage mark. To get to the mark, follow the directions in the troubleshooting guide here.