In certain situations your iPhone may display one of the following errors:
"Invalid SIM" or "No SIM Card installed" alert appears intermittently.
Status bar displays "No SIM" when a SIM card is installed.
It could be for any of the reasons below. Run through the checklist.
Remove the SIM Card and check that it is not damaged or worn. Then reinsert it.
Still no luck?? Then I’m afraid it’s time to get in contact with Apple. You can contact them a few ways, check here http://www.apple.com/support/contact/.