You mean your iPad has stopped recognizing your SIM card even though you have a data plan switched on? If resetting doesn't work, you probably need to talk to your carrier. Maybe there's something they can adjust on their end, or maybe you need a new SIM. Mine got corrupted once, and AT&T told me to get a freebie at an AT&T store.
If you have no data plan on (or have it temporarily switched off), just tap to ignore the message and use wi-fi, if you have that.
I often pull out my SIM, because I have an unlimited data plan and switch it between two iPads. So I regularly get a "no SIM card" message on the one without a SIM card. I just ignore, tap the screen and go about my business on wi-fi. I need only one SIM card, because I only take one iPad out of the house at a time. I just take whichever is fully charged.
