IPhone 4S showing SIM Failure message?

My iPhone 4S is just over 6 months old on O2-UK and today it came up with a message saying "SIM Failure" when I wasn't using it. The signal bars went completely, and then it then said "SIM Invalid" in the top left hand corner where the signal should be.

Has anyone else had this problem? Is the phone or SIM card to fault?

Should I take the phone back to Carphone Warehouse?

Turn off your phone, eject the SIM tray, and remove the SIM card. Wipe the contacts with a cloth, then reinsert and restart your phone.

If that doesn't resolve the issue, it's time to bring it back to where you bought it.

Do what the other person said.

Try placing the O2 sim card in another phone, if you have one available. This will determine whether it is the sim card, or the phone.

If not, take the phone to Apple, O2 - or the store you bought it in, and get them to take a look at it. Hard to tell whether it could be the sim card or phone. Whatever it is, you'll get a replacement/fix for free because the phone is still in warranty if it's less than a year you've had it, and sim cards aren't worth anything.