I inserted a working sim card but iPad(2) says "no SIM" - cellular data setting greyed out

I inserted a working SIM card into my iPad 2. It picked up the network but later gave a message saying "No SIM". The Cellular Data control option is greyed out so I cannot get in to this function. I have tried removing the SIM and re-inserting as well as re-starting. This made no difference. Any suggestions why this is the case? I have tried the SIM in my iPhone 4 and it works fine. Thank you.