From time to time, your iPhone may stop connecting to 3G/4G networks, preventing you from making or receiving calls, or using wireless data. There are a number of reasons why this can happen, including that the iPhone doesn't recognize the SIM card.
You'll know this is happening a message will pop up on your iPhone screen warning of "No SIM Card Installed". You'll also notice that in the top left corner of the screen, the carrier name and signal bars/dots that are usually there have been replaced by "No SIM" or "Searching." In most cases, the problem is just that your SIM card has become slightly dislodged. Once you re-seat it, all should be well. And, luckily, fixing this problem requires no more than a paper clip.
Locating the SIM Card
At the top of the iPhone (original, 3G, 3GS models), between the hold button and the headphone jack is a slot with a tiny hole in it. This is the tray that holds the SIM card.
On the iPhone 4, 5, 5C, and 5S, the SIM tray is on the right side of the phone. These models use a microSIM, a slightly smaller, more modern version of the SIM card. As a result, SIMs from older phones won't work in newer models without modification.
Re-Seating the SIM Card
To re-seat the SIM card in its slot, get a paper clip (or, as Apple cheekily calls it, the "SIM card removal tool"), unfold it, and gently push one end into the hole in the SIM card tray. This will pop the tray out of its slot. Push it back in and make sure it's firmly seated.
After a few seconds (wait up to a minute or two), the warning should disappear and your regular bars and carrier name should re-appear in the top left corner of the iPhone's screen.
If it doesn't, eject the SIM again and take it out. Make sure it–-and the slot it sits in--aren't dirty. If they are, clean them (blowing into them is probably OK, but a shot of compressed air is always best) and reinsert the SIM.
Testing for a Malfunctioning SIM Card
If this still doesn't fix the problem, the SIM card could be malfunctioning. One good way to test this is to try inserting a SIM card from another cell phone (if you've got one handy) into your iPhone and seeing what happens (make sure to use the correct size--standard or microSIM--for your phone). If the No SIM warning disappears from the top left corner and signal bars reappear, then your iPhone SIM is broken.
Make Sure your SIM Is Valid
It's also possible that the SIM card in your iPhone isn't valid. SIMs can be assigned to particular phone companies and customer accounts. SIMs can be deactivated or not work at all in some countries. If you're still getting an error message, make sure the SIM you're using is valid. A good way to do this is to swap in another SIM that you know works.
If Nothing Works
If all of these steps dosn't solve the problem, it's likely that you've got a problem that you probably can't fix. It's time for a trip to your closest Apple Store or your phone company store, or to call tech support.
Got a tech support question? Ask it in the discussion forums.
Want tips like this delivered to your inbox every week? Subscribe to the free weekly iPhone/iPod email newsletter.