Why sim is not working in my iPad ?

Be aware that the majority of Phone SIMs are not correctly provisioned for most tablet computers - and most will offer no service at all with an iPad. In most cases, you will require a DataSIM that is explicitly intended for tablet computers and Cellular Routers (also known as MiFi).

If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

You will need to verify suitability of your SIM, for use with iPad, with your chosen Cellular Provider. You will obviously require an active Cellular plan, otherwise the SIM will likely be deactivated.

It's impossible to say since you've provided no information at all.