We will need a bit more information to help you. Such as, you have an iPad with Wi-Fi+GSM/Cellular or just Wi-Fi? Why/when did you start having this problem?

But I will start with some basic info. If you have an iPad with GSM/Cellular, and a SIM card is inserted, you will see the ICCID (it is the ID of the SIM card itself). If you do not have a SIM inserted, the iPad (or phone) will say ICCID is unknown.